The Zimbabwean striker's transfer had initially been held up after emails and faxes from the Stadium of Light to the Football Association failed to arrive before Monday's 5pm transfer deadline.
But after spending much of yesterday deliberating over the merits of Sunderland's claims, the Premier League finally ratified the temporary agreement 23 hours after a deal was supposed to have been concluded.
The Black Cats provided documentary evidence that their internet provider broke down in the closing stages of the transfer window, meaning Benjani's registration and medical documents failed to arrive in time.
A league spokesman said: The Premier League confirms everything was in order and deal done before the transfer window closed.
Sunderland were able to breathe a huge sigh of relief yesterday after the Premier League's investigations ended favourably; any other outcome would have further restricted manager Steve Bruce's options.
Having allowed Daryl Murphy and David Healy to join Ipswich on loan, Bruce's only experienced strikers with any first team experience were Kenwyne Jones, Darren Bent and Fraizer Campbell ahead of Saturday's match with Wigan.
Benjani, who has spent the last four years in England after moving from Auxerre, now offers Sunderland an alternative, with Bruce hoping he can recapture the sort of form that earned him 19 goals in 70 league appearances for Portsmouth.
His two years at Manchester City brought just four league goals, but Bruce is confident he can help the forward find his feet again.
Bruce said: He's a big, strong, powerful centre forward and one we've been tracking for a long time. I'm delighted that we've been able to bring him to the club.
He'll bring another dimension to our striking options and will be a great squad addition.'' The short-term acquisition of Benjani meant Bruce had added three new faces to his squad in the transfer window, following Alan Hutton and Matt Kilgallon.
The squad, however, has hardly been strengthened in terms of numbers, following the departures of Murphy, Healy and Nyron Nosworthy, who joined Sheffield United on deadline day until June.
And left-back George Mc- Cartney said: I was probably surprised myself to see those three lads go, with the squad being so small and the injuries we have had over the last few weeks.
But the manager clearly feels we have enough strength in depth to cope until all the injured lads come back. It will be nice to have a fully fit squad again.
I'm sure the manager attempted to sign other players and it never happened. It's a short period of time and we did manage to get one or two new faces in. Maybe he wanted others he didn't get.
McCartney was frustrated with Monday's goalless draw with Stoke City at the Stadium of Light, a result which leaves Sunderland too close to the relegation zone for comfort ahead of matches with Wigan and Portsmouth.
The Northern Ireland international is confident the new arrivals will help to turnaround a run of form that has recorded just one win from their last 14 Premier League games.
He said: Matt played against Everton and on Monday and he has had two solid displays; Alan Hutton is a good attacking full-back. He didn't play much at Spurs and now he will be looking to show what he is made of here.
I also know Benjani has had similar problems. He will need games and he will be looking to prove his worth during his time here. That should help us.
Salvador Cabanas could still make Paraguay's World Cup squad, insists one of the doctors treating the forward.
Cabanas, a Sunderland transfer target, was shot in the head in Mexico City just over a week ago in a city bar.
The striker was transported to an intensive care unit at a local hospital and rushed into theatre where doctors made an attempt to remove the bullet lodged in his skull before deciding it would be too dangerous.
Cabanas, who was initially given only a 20per cent chance of survival, is now beginning to regain movement in his arms and limbs and neurosurgeon Celso Fretes believes he has every chance of playing in the World Cup.