Apollo said it will merge ADT with its security and home monitoring firm Protection 1.
The combined company is expected to generate $4.2bn in annual revenue.
ADT's president said the purchase price offered investors a "highly attractive premium", while a partner from Apollo said the deal would enhance the fund's position in the fragmented industry.
Apollo's offer is 56% higher than the value of ADT's shares at the end of trading on 12 February.
ADT and Protection 1 both offer home and office security monitoring services. ADT serves more than 6.5 million clients while Protection 1 serves two million.
"This transaction provides the opportunity to dramatically enhance our position in the large, fragmented and growing residential and business interactive electronic monitoring industry," said Marc Becker, senior partner at Apollo.
ADT's board unanimously approved the deal. It now has 40 days to look for alternative offers, a standard practice in these types of transactions.
The deal is expected to be completed by June.
Private equity fund Apollo said on Tuesday, it plans to buy security company ADT for $6.9bn (£4.8bn).