Post-merger integration checklist: the first 100 days after closing

Integration plans fail for ordinary reasons. Nobody could approve a payment on Day 1. A key engineer left in week three because no one had talked to her. A customer heard about the deal from a competitor. The checklist below is built around those failure points, in the order they tend to bite.

What should be ready before closing?

Integration starts during diligence, not after it. Between signing and closing, the buyer usually has a window to prepare, subject to one important limit: until closing, the two companies are still separate, and the buyer must not take control early.

In the US, transactions that require a filing under the Hart-Scott-Rodino Act cannot be consummated before the waiting period ends, and the antitrust agencies treat premature coordination as “gun-jumping”. The FTC’s premerger notification program pages explain the regime. Similar rules apply under merger control in the EU, UK and elsewhere. Practical consequence: plan freely, share competitively sensitive information only through a clean team, and do not direct the target’s business before closing.

Before closing, aim to have:

• an integration lead and a small integration office with named workstream owners;
• a Day 1 plan covering finance, IT access, HR and communications;
• a list of contracts that need consent or notice on change of control;
• retention agreements for key people, ready to sign;
• draft messages for employees, customers, suppliers and, if relevant, regulators.

What belongs on the Day 1 checklist?

Day 1 is about control and continuity. Nothing ambitious; everything essential.

Payroll deserves a special mention. A missed or wrong payroll in the first month does more damage to trust than almost anything else an acquirer can do, and it is entirely preventable.

What happens in the first 30 days?

The goal for month one is stability. People, customers and operations should notice as little disruption as possible.

People. Meet key employees individually, not just in town halls. Confirm reporting lines and decision rights. Address the questions everyone asks, about jobs, pay and benefits, as early and as honestly as you can.

Customers. Contact the largest customers personally. Confirm that contracts, contacts and service levels continue. Watch for competitors using the deal to unsettle accounts.

Diligence follow-up. Turn the diligence findings into an issues register. Every material item, from an unresolved tax audit to an expired permit, gets an owner and a target date.

Governance. Stand up the integration office with a regular, short meeting, a single tracker and a clear escalation route.

What happens between day 31 and day 60?

With the business stable, integration begins in earnest.

• Policies and approvals. Align delegated authorities, procurement rules, expense policies and compliance procedures.
• First synergy actions. Start with the low-risk ones: procurement consolidation, overlapping subscriptions, insurance.
• Contract consents. Chase any counterparty consent that was not obtained before closing.
• Reporting. Bring the acquired business into the group’s monthly reporting, even if systems are not yet merged.

What should be done by day 100?

By the end of the first quarter, the integration should be running on its own rhythm.

• Systems roadmap. A sequenced plan for migrating finance, HR and IT systems, with realistic timings.
• Synergy tracking. Each target from the deal model linked to an action, an owner and a reported figure.
• Organization design. Final structure announced; open roles filled or being filled.
• Lessons learned. A short review of what diligence missed or got right, fed back into the next deal.
• The deal archive. Filed, indexed and under a retention policy.

For a buyer that acquires repeatedly, these 100 days become a playbook. Our guide to bolt-on acquisitions shows how platform buyers make integration repeatable.

How can the diligence data room help integration?

The diligence room holds the most complete, organized picture of the acquired business that will ever exist: contracts, permits, employee data, IT inventories, the Q&A log and the disclosure record. Integration teams usually rebuild that picture from scratch, which wastes weeks.

A better approach:

1. Freeze the disclosure record

   Export a complete, indexed archive of the room at signing or closing, including the Q&A log and audit trail, and store it unchanged as the legal record.

2. Create a separate integration workspace

   Copy only the documents integration teams need into a new workspace, so the frozen record is never altered.

3. Map folders to workstreams

   Give finance, HR, IT and legal integration owners access to their own folders, mirroring the diligence structure they already know.

4. Turn Q&A into the issues register

   Pull unresolved or caveated answers from the Q&A log into the integration tracker as actions with owners.

5. Add new material as it arrives

   Store consents, signed retention letters and migration plans alongside the original documents, so the history stays in one place.

6. Close the workspace when integration ends

   Archive it under the records policy and remove access once the integration office stands down.

Some teams simply keep the diligence room open for the integration period. That can work, but it blurs the line between what was disclosed before signing and what was added later, which matters if a warranty claim arises. Our article on how long a data room stays open covers closing and archiving a room properly.

What about personal data after closing?

Diligence rooms often hold employee and customer records. After closing, that data has a new controller and possibly a new purpose. Under the EU General Data Protection Regulation, personal data should be kept no longer than necessary for the purpose for which it is processed, so an integration workspace needs a stated retention period and access limited to people who need it. Privacy regimes in many other countries follow similar principles; our regional guides summarize them.

Which mistakes come up again and again?

From the many integrations that have been written up over the years, the same patterns repeat:

1. Treating Day 1 as a celebration rather than an operation. The announcement goes well; the payment run fails.
2. Ignoring culture until it becomes a retention problem. Different decision styles cause friction long before they appear in an engagement survey.
3. Chasing synergies before stabilizing. Cost cuts in month one unsettle the people the buyer most needs to keep.
4. Losing the diligence knowledge. The deal team moves on to the next transaction and the integration team starts from zero.
5. No single owner. Integration run by committee drifts.

None of these needs sophisticated tools. A named integration lead, a single tracker and a well-organized record of what was learned in diligence go a long way. The mergers and acquisitions hub covers the full transaction lifecycle, and the private equity guide looks at how sponsors run value-creation plans after closing.

How does integration differ for a carve-out?

When the acquired business is a carved-out division, Day 1 depends on the transitional services agreement. Payroll, IT and accounting may still run on the seller’s systems for months. The integration plan must include exit from each TSA service on a fixed timetable, or the buyer pays for services longer than planned. Our guide to carve-out due diligence explains how TSAs are scoped.