Ledley has been one of the star performers for Cardiff in recent years and stole the headlines once again when he netted the only goal of the game on 80 minutes.
Dave Jones' side can start dreaming about a possible place in the Premier League next season and put to bed the agony of missing out on the play-offs last season when Preston beat them by just one goal.
QPR are still not safe and manager Neil Warnock will be anxiously looking over his shoulder in the final two games of the season.
The home side could have opened the scoring after 18 minutes when the impressive Adel Taarabt raced towards the Cardiff penalty box and fired a low shot which cannoned back off the post.
Chances were few and far between in the first-half but Cardiff came close on the half hour mark when Stephen McPhail was unmarked outside the box and he drilled a 25-yard drive which flew just wide of the post.
Taarabt produced a great piece of skill on the stroke of half-time when he moved clear down the right.
The Moroccan, who is on-loan from Tottenham, played the ball across goal but Jay Simpson and Tamas Priskin somehow failed to connect with the ball.
QPR goalkeeper Radek Cerny almost gifted Cardiff the lead on 54 minutes when he fumbled a free-kick from Ross McCormack and the ball dropped into the path of Anthony Gerrard but he failed to get anywhere near the ball and Peter Ramage was able to clear the danger.
The home side blew another golden chance to break the deadlock on 65 minutes when Simpson flicked the ball into the path of Hogan Ephraim who was totally unmarked inside the box but he struck his shot over the bar.
Cardiff made them pay as Ledley scored the crucial match-winner with ten minutes to go.
Mark Kennedy broke away down the left and played the perfect cross into Ledley at the far post who made no mistake heading the ball past Cerny into the corner of the net.
QPR substitute Lee Cook could have salvaged an equaliser on 85 minutes when his close-range volley produced a superb save from David Marshall but the Welsh outfit held on for the win.