What to Expect from IT Integration During an M&A Deal

If you're an operations or finance leader going through your first acquisition, IT integration can feel like a black box. Lawyers and bankers handle the deal; then someone has to make two sets of systems work as one, usually on a tight timeline and with people who are already busy.

It's worth understanding, because a lot of the deal's value depends on it. Bain & Company found that more than half of business synergies are often contingent on systems integration, and PwC's 2026 M&A Integration Survey found only about one in three acquirers fully achieved all of their prioritized deal objectives.

Here's what the process actually looks like, phase by phase.

Phase 1: Due diligence (before you sign)

This is your chance to find out what you're buying before it's yours. Good IT diligence answers three questions: what's there, what will it cost to integrate, and what could hurt us?

Inventory: devices, servers, sites, internet connections, Microsoft 365 or Google tenants, and key business applications.
Security posture: past incidents, MFA coverage, endpoint protection, patching, backups, and who has remote access.
Contracts and licenses: which software licenses and contracts can transfer, and which have change-of-control clauses.
Cost: a realistic integration budget. EY-Parthenon's 2024 analysis found one-time integration costs typically range from 1% to 4% of deal value, and Bain notes IT integration estimates commonly run 20% to 50% over.

Diligence is also where security problems should surface. Verizon cut $350 million from its price for Yahoo in 2017 after Yahoo disclosed major breaches. Finding issues before signing gives you leverage; finding them after gives you a bill.

Phase 2: Planning (signing to close)

Between signing and close, the work shifts to planning. Three things matter most:

Decide the end state. Which Microsoft 365 tenant survives, which systems are kept or retired, and what security baseline everyone has to meet.
Write the Day 1 plan. What each group of employees needs to reach on the first day, who to call for help, and a step-by-step cutover runbook with a dry run before close weekend.
Negotiate the Transition Services Agreement (TSA). If the seller will keep providing some IT services after close, the TSA spells out which ones, for how long, and at what cost. Deloitte notes IT TSAs often last 8 to 12 months or longer, and vague service descriptions are a common source of disputes.

Phase 3: Day 1 (close weekend)

Day 1 has one job: people can work on Monday, and the environment is secure from the first hour. It's not the day to migrate everything.

A good Day 1 looks like this:

Every user can sign in, get email, and reach the applications they use daily.
Calendars and free/busy information work across both companies.
Privileged and shared passwords are rotated, and MFA is enforced on every account.
Departing employees' access is shut off.
Acquired staff get a clear message about what's changing and how to get help.

Bain's research found that the best acquirers plan Day 1 in detail ahead of time, and that email and intranet can often stay separate or loosely connected at first. That's fine. Stability comes first.

Phase 4: The first 90 days

With Day 1 behind you, the next three months are about stabilizing, securing, and starting to consolidate:

Days 1–30: confirm the real device inventory, get monitoring and endpoint protection on every device, and fix critical vulnerabilities.
Days 31–60: migrate mailboxes and OneDrive data in planned waves, starting with a pilot group.
Days 61–90: move the email domain to the target tenant, consolidate sign-in into one identity system, cut duplicate licenses, and start exiting TSA services.

A note on Microsoft 365: Microsoft's native cross-tenant migration tools need a one-time add-on license per user, a domain can only live in one tenant at a time, and some things (Teams meeting links, email signatures, Microsoft 365 Groups) have to be rebuilt after the move. Plan for those up front so they don't become day-60 surprises.

Phase 5: Full integration (months 4–18)

The remaining work takes longer: consolidating duplicate business applications, merging networks, unifying backup and disaster recovery, and exiting the last of the TSA services. McKinsey puts typical integration timelines for two organizations at 12 to 18 months, and Bain found the best acquirers complete about half of systems integration in the first year.

This is also where you measure results against the synergy targets set during diligence, and where the decisions made in Phase 2 pay off or come back to bite.

What separates smooth integrations from painful ones

After a lot of these, the pattern is consistent:

IT is at the table early, during diligence, not after close.
Day 1 is about continuity, not migration.
Security is handled on close, not "once things settle down."
One person owns the integration and can make decisions quickly.
The TSA exit is planned as carefully as the TSA itself.

Want the step-by-step version? Download our M&A IT Integration Checklist, or book a call and we'll walk through your deal.

Sources

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