It was announced in April that Lampitt would be leaving the Football Association after seven years, with his most recent post being head of integrity.
The 35-year-old replaces Peter Storrie, who stepped down from his position in March with the debt-ridden club in administration and facing inevitable relegation to the Championship.
Lampitt told the club's website: "I am excited to be coming on board today after the FA kindly agreed to release me early from my notice period.
"I know the club has been through some tough times, especially in the last season, but I am determined to help turn things around and bring some stability back to the business.
"I can't promise a quick fix but I genuinely believe we can put the club back on its feet.
"We will all have a part to play in this, the club staff and the playing squad, of course, but also, and probably most importantly, our extraordinary and loyal supporters.
"They have borne the brunt of the off-pitch problems over the last season with humility and stoicism, characteristics that I would like to see reflected in the club.
"I will be working extremely hard to rebuild their trust and, given time, I know that we can restore pride in an incredibly proud football club with proud traditions and a long history."
Administrator Andrew Andronikou remains in charge of Portsmouth and his company, UHY Hacker Young, released a proposed Company Voluntary Arrangement last week offering the club's creditors 20p in the pound.
But even that would see a total of £16.5million owed to non-football creditors, which would be paid over a five-year period, with the first £3million due during the nine-month period of the CVA.
Andronikou's other primary concern is trimming Portsmouth's wage bill, and he revealed Nadir Belhadj and Kevin-Prince Boateng are the two principal players the club are looking to move on.
The arrival of Lampitt means Portsmouth can turn their attention to finding a new manager following Avram Grant's departure last month.
"There is a lot to do," Lampitt added. "My initial priority, in addition to finding a new manager and taking steps to secure our squad for next season, must of course be to ensure that the CVA proposals sent out last week are agreed by our creditors on June 17 so that the future of the club is secured.
"During the continuing period of administration the administrators remain in full legal control of the club and I will be working alongside them to make sure that the interests of the club are protected."