The Brit, fighting in his homeland for the first time since 2013, put the 33-year-old down after catching him with a sharp right-left combination only 13 seconds in.
Khan was in terrific form from the off against Phil Lo Greco in Liverpool as the Bolton man took two-years of frustration out on the hapless Canadian at the Echo Arena.
"Khan is back", Khan said afterward in the ring.
Khan then pinned Lo Greco to the ropes before the superwelterweight contest ended moments after it had started.
Khan had not fought since a knockout loss to Saul "Canelo" Alvarez in May 2016, but showed no lingering mental scars as he ruthlessly attacked Lo Greco from the opening bell. In 2009, he won the WBA light-welterweight title and followed it up with the IBF title in 2011.
Khan also tweeted about the fight. "My target is to win another world title", Khan was quoted as saying by ESPN after his win.
"The future holds some big fights".
Amir, come on, Amir, all the fans want the fight.
When asked if he is targeting a fight against Brook, Khan said: "I want to become a world champion again so I want to fight the top guys in the welterweight division". This is where me and Eddie need to sit down with Kell's team. That fight I'm not rushing and I'm not running away from.
"The weight is an issue with Kell [Brook]". I want the fight.
"The UK market is massive now and fighters from America want to come here". I'm not Lo Greco, I'm Kell Brook, "The Special One" and I will deal with him, I'm a seasoned pro, I will land that bomb on him, that brownie.
Broner fought out a majority draw during an entertaining bout against Vargas in NY last night, with one judge scoring the contest 115-113 for Broner and two scorecards of 114-114.