The Three Lions travel to take on the Czech Republic in Prague on October 11 before a trip to face Bulgaria three days later.
Tomori and Abraham were two of three players included this time around who had missed out on the previous squad, with Everton midfielder Fabian Delph also recalled.
Manchester United pair Jesse Lingard and Aaron Wan-Bissaka drop out, along with Alex Oxlade-Chamberlain of Liverpool.
Tomori, who was born in Canada and represented them at Under-20 level, has established himself in Frank Lampard’s Chelsea side of late and is rewarded with a maiden senior England call-up.
Abraham scored his eighth goal in 10 games in Chelsea’s 2-1 Champions League win over Lille on Wednesday night and is back in the squad having made his debut almost two years ago.
He featured in friendly matches against Germany and Brazil in November 2017 and remains eligible for Nigeria unless he picks up a competitive cap in the forthcoming games.
Manchester City right-back Kyle Walker is again a high-profile absentee, with Kieran Trippier and Trent Alexander-Arnold preferred.
Asked if it was an easy decision to include Abraham, Southgate said: “It was – he’s the leading English scorer in the league.
“He’s a player that came into the squad very young, at a time when we wanted to have a look at players ahead of the World Cup.
“That was a bit early for him but now with the opportunity he’s been given by Frank, along with other English players at Chelsea, it’s a great chance for Tammy.
“He’s always scored goals with our junior teams, with the Chelsea youth teams, on loan in the Championship and now he’s really grasped this opportunity at the beginning of the season.”
Abraham remains eligible for Nigeria until he makes a competitive appearance for the Three Lions, but Southgate denied the dual-nationality issue influenced his inclusion or that of Tomori
“I haven’t discussed that with him, I’ve left it to Frank to have the conversation,” he said.
“I don’t think it’s ethically right to select someone just to make sure he can’t play for somebody else – Fik and Tammy are in on merit.
“They’ve both been part of our youth system, Fik came in a bit later but he was part of the team that won the (Under-17) World Cup – as far as we’re concerned they’re English players.”
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