Middlesbrough's narrow victory - their first in five attempts - came courtesy of Julio Arca's 40th-minute penalty after Gabor Gyepes had brought down Marvin Emnes, and the result propelled the hosts out of the bottom three.
But Cardiff, who rescued an injury-time point at home to bottom club Preston last weekend, have seen their promotion charge reduced to more of a lame jog with just one win in their last seven games.
That sequence has seen frustration creep in among some supporters, with boss Dave Jones this week calling for all quarters to stick together while also having to bat away claims of internal disharmony within his ranks.
Amazingly, however, City remain second in the table, just four points off leaders QPR who were undone 3-1 at home to Watford on Friday night.
The Welsh outfit were without 13-goal top scorer Jay Bothroyd due to a hamstring injury, with Andy Keogh - whose last-gasp header salvaged a 1-1 draw against North End seven days ago - starting in attack alongside Craig Bellamy.
Bluebirds centre-back Gyepes and on-loan midfielder Danny Drinkwater also came in.
Middlesbrough's starting XI showed two defensive changes from the side who lost 1-0 at Coventry, Seb Hines and Justin Hoyte taking the places of the suspended David Wheater and dropped Tony McMahon.
And it was Boro's backline who were put under early pressure by a bright Bluebirds start, although it did not take long for the hosts to settle.
Former Swansea loan striker Emnes forced Cardiff keeper David Marshall into a routine save after 14 minutes before strike partner Scott McDonald poked harmlessly wide.
But, despite a strong work-rate from both sides, there was very little goalmouth action of note inside the opening quarter.
That trait continued as the opening 45 minutes wore on with the pace of proceedings also dropping dramatically.
Patient build-up play saw McDonald find the side-netting from a tight angle just before the half-hour mark, with Boro posing more of a threat up front despite City's array of attacking riches.
With the half-time whistle appearing a welcome relief, a spark finally arrived when Gyepes brought down Emnes in the area, leaving Arca with the simple task of sending Marshall the wrong way from the spot.
You got the sense a second goal would kill off Cardiff and Nicky Bailey saw his powerful drive palmed away by Marshall at the start of the second half as Boro went on the offensive.
Jones introduced Michael Chopra in the hope of adding impetus to his attack and it did eventually materialise.
Bellamy fired a difficult first-time effort into the side-netting in the 68th minute before clipping the top of the crossbar moments later with a fine drive.
Jason Steele then turned Chopra's chip to safety as the pace picked up but the visitors' late pressure was not enough, leaving Middlesbrough celebrating their first win over Cardiff since 1984.