Mr McLachlan will become managing director and chief executive on August 5, Tabcorp said in a statement to the ASX on Monday.
He will act in an observer capacity until the regulatory approvals are made.
Mr McLachlan left the AFL in 2023 after a decade in the top job.
In the statement he said he was looking forward to the challenge of growing the gambling company's business by creating "entertainment for our customers in a safe way".
He said Tabcorp being Australian racing's biggest partner, contributing $1.1 billion in 2023, and a presence in all states but WA was an important component in his decision.
Mr McLachlan will get $1.5 million a year inclusive of superannuation as his base income.
He will be eligible for short-term and long-term bonuses and subject to a 12-month non-compete clause at the end of his employment.
Mr McLachlan joined the AFL in 2000 and was appointed chief operating officer in 2008, then deputy chief executive four years alter.
He succeeded Andrew Demetriou as chief executive in 2014, overseeing the creation of the AFLW and the AFL's COVID-19 response.
He announced his departure in 2022, but stayed on into 2023 at the request of the AFL.
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