He lost his first pro fight and then became ATG

Henry Armstrong

 

Benny Leonard ended up pretty good.

 

Miguel Canto
Manuel Ortiz
Ted Kid Lewis

 

There are probably a lot of hipsters on here who don't think JMM is an ATG but I'll say JMM because he simply is.

 

Lol which clown thinks Marquez isn't an ATG show yourself

 

Not quite ATG, but it's interesting that his brother also got himself a good career coming off a loss in first career fight.

 

Juan Manuel Marquez
Henry Armstrong
Benny Leonard

 

Heavyweight great, Jack Dempsey, had a draw in his first recorded match, not a loss, but still, with a different referee, it sure might have been...

EDIT: This is according to Wikipedia, not BoxRec. He was probably fighting as Kid Blackie for that Draw.

 

Lloyd Marshall

 

Jack Britton won his first fight but lost the next two and was kayoed for the only time in his career in the second of those two losses. In inauspicious start for an indisputable ATG.

 

Old Bones Brown's first three fights were an NC, a draw and a KO loss.

 

No mention of Miguel Canto ? Y'all should be ashamed of yourselves.

 

Not an ATG but certainly a notable, James “Bonecrusher” Smith was stopped in four rounds by a body shot from James Broad (a highly decorated amateur and prospect) in his debut.

In his second fight, he beat future cruiserweight champ Ricky Parkey.

 

It seems that several non-ATGs are being mentioned here... so if we want to lower the bar, and include those who fall just short of ATG status, there's plenty to choose from. Some examples could be:

Lew Tendler
Eddie Perkins
Willie Ritchie
Bruno Arcari
Pipino Cuevas
Gorilla Jones
Carlos Teo Cruz
Tommy Freeman
Johnny Nelson

... and the list goes on and on (more than 60 world champs lost their pro debut!)