The result left Serbia second in the section on eight points from four games, two behind leaders Ireland who were 1-0 winners in Austria. Wales are third on six points while fourth-placed Austria have four.
Having fired the Welsh ahead in the 30th minute, Bale hit the inside of the post moments before Newcastle United striker Mitrovic equalised when he got away from his marker to head home Antonio Rukavina's cross from the right.
Serbia had the upper hand early on as captain Branislav Ivanovic saw his header cleared off the line by Neil Taylor and Ashley Williams blocked Dusan Tadic's fierce shot from the edge of the penalty area.
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Subdued by a packed Serbian midfield, Bale showed his class in the 30th minute when he scored his 26th international goal to close in on Welsh all-time leading scorer Ian Rush (28).
Hal Robson-Kanu won the ball off Matija Nastasic by the touchline and squared it for Bale, who beat keeper Vladimir Stojkovic with a crisp low shot from 18 metres.
The visitors came close on the stroke of halftime when Wales keeper Wayne Hennessey parried a ferocious Nemanja Matic drive from 25 metres, after Robson-Kanu had headed a Bale cross wide of the far post at the other end.
It was a scrappy contest for much of the second half after Bale tested Stojkovic with a dipping free kick from 30 metres in the 50th minute but though Serbia pressed forward, they failed to create any clear-cut chances.
With Rush an interested spectator, Bale should have made it 2-0 in the 85th minute but his shot rebounded off the woodwork after he was put through by Aaron Ramsey.
Wales paid dearly for the miss as Serbia broke forward and silenced a passionate home crowd in Cardiff City Stadium.
Wing-back Rukavina, thriving on the right flank in a revamped 3-4-3 formation, delivered an inviting cross and Mitrovic twisted away to send it in off the far post and Hennessey, who could not keep it out.
Wales next visit Ireland on March 24, with Serbia away to Georgia and Austria at home to Moldova.
(Writing by Zoran Milosavljevic; Editing by Ian Chadband)
