Most post-merger IT integrations don’t fail because of one big mistake. They fail because of fifty small ones that nobody tracked. A TSA you forgot to negotiate before signing. A licence true-up you discovered the hard way. A data migration that was mostly cleansing, not loading, and nobody budgeted for it.
That’s why this checklist exists. It turns the overwhelming job of post-merger IT integration into a single scannable master tracker you can run against every phase of the deal from signing through steady state. Assign an owner, a target date, and a RAG status to each item, and you’ll catch the failures before they compound.
The numbers behind it: 70% of mergers fail to hit their revenue synergy targets, with IT integration delays the primary culprit in more than half of cases (McKinsey). IT integration consumes 25 to 40% of total integration cost, more than any other workstream (Deloitte). And for carve-outs, IT separation exceeding 18 months underperforms the investment thesis by an average of 340 basis points of IRR (Journal of Private Equity).
Here is the checklist, organised into five phases. Use it as your working tracker.
Phase 0: Pre-Close Checklist (Signing to Close)
Decisions made before close determine everything after it. Every month of pre-close planning saves 2 to 3 months of post-close execution.
- IT integration lead appointed named no later than signing, with authority over architecture and budget
- Clean team established if carve-out or competitive situation, to access seller IT data safely
- High-level IT integration strategy defined which archetype (absorption, preservation, symbiosis, carve-out)
- Target application inventory completed including shadow IT and embedded systems
- Infrastructure dependency map created networks, servers, storage, cloud
- Shared service dependencies identified critical for carve-outs (shared ERP, AD, data centres)
- High-level cost estimate developed at plus or minus 30% accuracy
- Vendor and licence impact assessment completed renegotiation strategy for enterprise contracts
- IT key-person retention plan in place the people who know the legacy systems
- Day 1 readiness checklist defined what must work the moment the deal closes
- Integration governance structure agreed who decides, escalation paths
- TSA terms reviewed and negotiated duration, scope, pricing, exit provisions, data deletion (carve-outs)
Phase 1: First 100 Days Checklist (Month 1 to 3)
Day 1 Readiness (0 to 48 Hours)
- Email and calendar continuity for all employees
- Network connectivity and internet access active
- Identity and access working employees can log in to core systems
- Payroll and HR systems operational
- Customer-facing systems up website, support portal, order systems
- Security baseline active firewall, endpoint protection, MFA
Discovery and Foundation (Days 2 to 100)
- Detailed application discovery completed full catalogue with dependencies mapped
- Infrastructure topology fully mapped
- Data inventory and classification completed volumes, sensitivity, retention requirements
- Target architecture blueprint finalised
- Migration sequencing plan approved what moves when, in what order
- IT integration team fully staffed infrastructure, applications, data, security, service management
- External partners onboarded system integrators, migration vendors
- Integration war room and tooling established
- Day 1 systems confirmed operational
- Email and collaboration baseline deployed M365 or Google Workspace across both orgs
- Interim security controls implemented MFA, endpoint protection
- Detailed budget approved at plus or minus 15% accuracy
Phase 2: Build and Migration Checklist (Month 4 to 12)
The heavy lifting. This phase consumes 55 to 65% of the total IT integration budget.
- Network integration or separation completed WAN, LAN, VPN, firewalls, SD-WAN
- Data centre or cloud migration executed
- Application rationalisation decisions made apply Gartner’s TIME model (Tolerate, Invest, Migrate, Eliminate)
- ERP migration or replacement in progress or deliberately deferred to hold period
- Data migration cycle 1 completed and validated budget about 40% of effort for cleansing, not loading
- Identity management consolidated single source of truth for users
- Single sign-on (SSO) implemented
- Security monitoring integrated across both environments
- Service desk transitioned ITIL 4-aligned incident, problem, change management
- User acceptance testing completed for each wave
- Cutover plan approved with business continuity safeguards
- Rollback procedures tested so a failed cutover doesn’t become a crisis
Phase 3: Stabilisation Checklist (Month 10 to 18)
Where the integration proves itself, and where carve-outs exit the TSA.
- All integration defects tracked and resolved
- Performance baselines meeting targets
- User training completed adoption programmes, not just documentation
- Legacy systems fully decommissioned
- Documentation handed over to BAU teams
- TSA exit completed for carve-outs, the formal handover from seller IT
- Seller data certified as deleted for carve-outs, with GDPR implications
- Business continuity tested on standalone environment
- Integration programme formally closed
Phase 4: Steady State Checklist (Month 18 and beyond)
- IT operations transitioned to BAU teams
- Synergy tracking dashboard operational measure against the business case
- Continuous improvement backlog prioritised
- Residual technical debt documented so it isn’t invisible
- Lessons learned captured and shared
- Technology roadmap for the hold period defined
The Five Most Expensive Items People Skip
If your checklist is short, make sure it still includes these. They are the five items most likely to blow a hole in your budget or timeline when overlooked.
- Licence true-up. Separating from a parent’s enterprise agreement can increase per-unit licence costs 2 to 3x. Budget GBP 100K to 500K for mid-market deals.
- TSA extension costs. Missing TSA exit costs 50K to 200K per month depending on services, and erodes seller goodwill.
- Data cleansing effort. Assume 40% of data migration is cleansing and transformation, not loading. It is the most consistently under-budgeted line item.
- Shadow IT remediation. You’ll find applications nobody disclosed. Budget 10 to 15% contingency specifically for them.
- Data centre exit fees. Early termination or migration fees on shared hosting are routinely missed during diligence.
How to Use This Checklist
Run this checklist as a living document, not a one-time audit. For each item assign an owner, a target date, and a RAG status (Red / Amber / Green), and review it in the integration war room every week. The items that slip to Red are your early-warning signal. Fix them before they become integration failures.
For carve-outs specifically, treat the TSA as your hard deadline: every item in Phase 3 that isn’t complete by your TSA exit date has a direct monthly cost attached to it.
How AssetMax Supports Post-Merger IT Integration
AssetMax builds CarveX specifically for the hardest integration scenario, the corporate carve-out. It maps your IT landscape in 5 to 10 days, identifies every TSA dependency before you sign, and models the optimal migration sequence to minimise disruption. For PE investors, that translates to 30 to 50% faster TSA exit and 15 to 25% lower separation costs.
Pair it with Diligize for technology due diligence that shapes the deal, and Praetorian for continuous security monitoring while networks and identities are in flux.
Frequently Asked Questions
What is a post-merger IT integration checklist?
A post-merger IT integration checklist is a phased list of the IT tasks required to combine two technology estates after a merger or acquisition, organised from pre-close planning through steady state. It covers infrastructure, applications, data, security, identity, and service management, with owners and status tracking for each item.
How many items should a post-merger IT integration checklist have?
A comprehensive checklist runs 50 to 60 items across five phases (pre-close, first 100 days, build and migration, stabilisation, and steady state). The exact count matters less than coverage: every phase, every workstream, and every hidden cost must be represented.
What is the difference between IT integration for a merger vs. a carve-out?
A merger combines two systems; a carve-out extracts one system from a shared parent environment under a Transitional Service Agreement (TSA) with a hard deadline. Carve-outs carry a 3 to 5x complexity multiplier because shared infrastructure must be replicated, not just joined.
How long does post-merger IT integration take?
Standard mergers typically take 12 to 18 months to reach steady state. Carve-outs with ERP separation routinely take 18 to 24 months, driven by the TSA clock and the need to build a standalone environment.
What are the most common IT integration mistakes?
The five repeats are: treating IT as a post-close problem, assuming data migration equals integration, underestimating complexity, losing key IT talent during transition, and integration fatigue. Each is preventable with early planning and a disciplined checklist.
For the full framework behind this checklist, timelines, cost benchmarks, and carve-out guidance, see our complete guide to post-merger IT integration.