The 20-year-old headed in fellow substitute Hasney Aljofree's cross eight minutes from time.
But the Pilgrims were indebted to keeper Luke McCormick, who produced a fine display to keep Coventry at bay.
The 22-year-old tipped a 20-yard Stern John effort over in the first-half and made another acrobatic save from Colin Cameron's free-kick after the break.
Plymouth boss Ian Holloway:
"I am really pleased for Samba. "He has been in Spain for the last three or four years and we beat off other clubs to sign him.
"He has had to be patient for a game but plays with a smile on his face and has been a breath of fresh air around the place. I'm really pleased for him."
Coventry: Marshall, Whing, Page, Ward, Hall, Birchall (Adebola 65), Cameron, Hughes, Tabb (Hutchison 73), Kyle, John.
Subs Not Used: Doyle, Heath, Thornton.
Booked: Cameron, Page.
Plymouth: McCormick, Connolly, Seip, Doumbe, Capaldi, Norris, Wotton, Nalis, Buzsaky (Aljofree 78), Chadwick (Summerfield 85), Reid (Samba 74).
Subs Not Used: Larrieu, Sawyer.
Booked: Norris.
Goals: Samba 82.
Att: 19,545
Ref: M Halsey (Lancashire).
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