Bill Hamid made one save for D.C. United, who returned from a three-week international break on the heels of posting a 3-0 victory over Inter Miami CF on May 29.
John McCarthy finished with two saves for Inter Miami, who have been shut out in three straight matches and five times in their last seven.
Inter Miami's Gonzalo Higuain did not compete in Saturday's match after he "stayed back working on his fitness," the South Florida Sun Sentinel reported via a club spokesman. Higuain leads the team with four goals on the season.
D.C. United took advantage of a hand ball in the penalty area by Inter Miami's Christian Makoun to open the scoring. Kamara, who scored twice in the clubs' last meeting, hesitated slightly before wiring a right-footed shot into the right corner of the net for his team-leading fourth goal of the season.
Hamid made the slim advantage stand up en route to recording his second straight shutout and 72nd of his career. The 30-year-old has competed in just two matches since dealing with a calf strain on the heels of undergoing offseason hernia surgery.
Kamara was unable to convert three prime opportunities during a scoreless first half. He was denied in both the sixth and 42nd minutes before his header following a set piece in the 45th caromed off the left post.
Ryan Shawcross also received a red card in the 84th minute, limiting Inter Miami to nine players for the remainder of the match.