Charlton's draw with Wigan meant that Boro were safe, should the Latics 12-point penalty be enforced for entering administration and the Lancastrians lose their appeal.
The bad news arrived with their contagious homesickness continuing. Boro have not won on Teesside this decade - with their embarrassing run without a home victory, which stretches back to Boxing Day, extending to 11 matches in all competitions.
Neil Warnock's former club ran out easy victors as Boro - who saw Jonny Howson dismissed late on for a second yellow card - stumbled to a sixth loss in seven at the Riverside.
Horrendous is the phrase and all will take all of Warnock's know-how to sort out the Teessiders should he stay put next season.
Warnock warned ahead of the game of Cardiff being a stronger team than the previous two sides who had gone away from the Riverside with three points in Bristol City and QPR, while referencing the reliability of those in blue who he knew intimately from his time in the Principality.
It took just four minutes for the Bluebirds to remind Warnock of what he had left behind, with the Boro chief - not for the first time in his reign - left to lament his side's haphazard defending.
It was a classic Warnock set-piece goal, with a beautifully flighted corner from Joe Ralls finding Sean Morrison in splendid isolation and with Boro caught ball-watching, the big centre-half headed home from his sort of range following school-boy defending from the hosts.
If that concession was bad, the second goal from Cardiff was simply atrocious from a Middlesbrough perspective and will have had Warnock tearing his hair out in the 47th minute when his side were caught with their pants down..
The goal started on the edge of the Cardiff box with Boro on the attack, with the ball finding its way to former Boro favourite Tomlin who exploited the ocean of space on the left vacated by Anfernee Dijksteel to send substitute Josh Murphy clear and he kept his cool to steer the ball low past the advancing Pears.
Going forward, Cardiff possessed much the bigger threat, with Robert Glatzel, a player brought to Wales by Warnock, showing the managerial veteran just why he wanted to bring him to the club from Germany.
Big, strong and aggressive, he cashed in on an error from Dael Fry to surge towards goal on 19 minutes, only to be brought down by a shirt tug by the chasing George Friend, with only the fact that Fry had tracked back managing to save him from a red card.
Friend was in final warning territory after catching Glatzel later on in the half, one that Cardiff largely controlled against a home team who named just seven substitutes.
Boro huffed and puffed, but their threat in the final third was minimal, with Patrick Roberts struggling to recreate his magic of midweek.
The chances were in front of the Boro goal, with Aynsley Pears turning away a fierce strike from Ralls and Leandro Bacuna firing wide on the stretch for a Cardiff side who featured another ex-Boro player in Joe Bennett alongside Tomlin and another on the bench in Aden Flint.
An interval deficit became a mountain to climb soon after the resumption after Dijksteel's poor decision-making left Boro horribly exposed and Murphy was not about to look a gifthorse in the mouth.
Warnock made a triple substitution in a bid to bring some life to an aimless display with Britt Assombalonga, Hayden Coulson and Paddy McNair all called for.
A lifeline almost arrived midway through the half when Marcus Tavernier - their best player on the day by some distance - cut inside and saw his fierce strike rattle the crossbar before the underemployed Alex Smithies made a sharp reaction save to deny substitute Lukas Nmecha, who had come on for the quiet Ashley Fletcher.
Cardiff added a late third when Nathan Mendez-Laing got away from Fry down the right and his low cross found Murphy, with the rebound deflecting off him and into the net after Pears.
A consolation with a small c arrived for Boro courtesy of an excellent curling free-kick from Assombalonga, a moment of quality not in keeping with the hosts' forgettable performance as the striker registered his tenth goal of the season.
In the final minute, Howson picked up his second caution for a late challenge on Will Vaulks.
Middlesbrough: Pears; Dijksteel (Assombalonga 58), Fry, Friend, Spence, Howson, Saville (McNair 58), Johnson; Tavernier, Roberts (Coulson 58), Fletcher (Nmecha 67). Substitutes unused: Stojanovic, Wood, Wing.
Cardiff City: Smithies; Bacuna, Morrison, Nelson, Bennett; Mendez-Laing (Sanderson 90), Vaulks, Ralls (Bamba 90), Hoilett (Murphy 17); Tomlin (Pack 72), Glatzel (Ward 72). Substitutes unused: Etheridge, Flint, Smith, Paterson.