The 27-year-old started his Olympic Swimming career as a 15-year-old in Sydney in 2000, where the youngster was fifth in the 200m Butterfly.
Six gold and two bronze medals followed in Athens four years later before his historic eight gold medals in Beijing in 2008.
His Games in London did not start off so well with fourth place in the 400m Individual Medley - his worst result since Sydney - and he was shocked into second by Chad le Clos in the 200m Butterfly.
However, the Baltimore swimmer won the 100m Butterfly and 200m Individual Medley - where he outgunned team-mate Ryan Lochte - as well as taking the titles in the 4 x 200m Freestyle and Medley Relay and silver in the 4 x 100m Freestyle.
Texts and phone calls came from the likes of USA president Barack Obama and last night he was presented with a FINA lifetime achievement award by president Julio Maglione inscribed with the words: 'To Michael Phelps, the greatest Olympic athlete of all time. From FINA. August 4 2012. London, Great Britain.'
At a packed press conference afterwards, Phelps revealed what he had said to Bob Bowman, who had coached him since the age of 11, when he completed his final warm-up.
Phelps told Bowman: 'I have looked up to Michael Jordan all my life. He became the best Basketball player there ever was. I've been able to become the best swimmer of all time, we got here together. Thank you.'
Phelps now intends to travel and remain heavily involved with his Swimming foundation.
Of how he was feeling after his final swim, Phelps said: 'It's hard to put into words right now. I did everything I wanted to and finished my career how I wanted to.
'I've always said that I don't care what anybody else says, if I can say that about my career that's all that matters.'
While Phelps's retirement commanded the spotlight, the rest of his USA team-mates enjoyed great success, heading the medal table with 30 medals: 16 golds, eight silver and six bronze, as well as setting five of the nine world records.
Phelps claimed six medals, of which four were gold, while his Baltimore team-mate Allison Schmitt also visited the podium six times, topping it on three occasions.
Missy Franklin, 17, won five medals - four of them gold - while Ryan Lochte will also take home five, including two titles.
Katie Ledecky made an astonishing senior debut, winning the 800m Freestyle with the second fastest swim in history at the age of just 15.
Fellow 15-year-old Ruta Meilutyte of Lithuania also topped the podium, shocking herself with her 100m Breaststroke victory.
Chinese swimmer Ye Shiwen won the 400m Individual Medley gold in world-record time, and returned to win the shorter medley.
Fellow Chinese Sun Yang also set a world record, lowering his own mark in the 1500m Freestyle as he continues to steer the event into uncharted territory. It was his second title, and fourth medal, of the week.
The French enjoyed success: Yannick Agnel anchoring the sprint relay to victory in the final metres over the USA before taking the 200m Freestyle title. Camille Muffat became only the second Frenchwoman to claim an Olympic Swimming title when she won the 400m Freestyle, then finishing second over four lengths.
Ranomi Kromowidjojo won the sprint Freestyle double for the Netherlands but the Germans left without having visited the rostrum.
There were surprises. James Magnussen - 'the Missile' - did not claim the 100m Freestyle as had been widely predicted for much of the year, defending champion Britta Steffen did not make the final of the women's equivalent while Sarah Sjostrom, the Swedish teenager tipped for stardom, must wait for such success.
Most of all, though, the week in the pool was about Phelps, the like of whom we are unlikely to see again.
Elsewhere, the 10k Marathon saw men's and women's events in the Serpentine, with Tunisia's Oussama Mellouli and Eva Risztov of Hungary emerging victorious.
Mellouli became the first swimmer to win an Olympic medal in the pool and open water at the same Games after his bronze in the 1500m Freestyle.