The Wanderers led 20-14 at half-time and the scores were level at 30-30 at the end of normal time. But a last gasp penalty gave ex-Cardiff Blue McCarthy the chance to clinch a famous victory and the confident No10 hit the mark with his money shot to win 33-30.
It was a day of shocks all round as thew top three teams from last season were all sunk.
Bridgend were toppled at home by Ebbw Vale, 16-7, while cup holders Pontypridd were smashed at Sardis Road, 33-13, by Llandovery.
Scrum-half Bryan Shelbourne was Ebbw Vale's hero with 11 points at the Brewery Field, while Llandovery out-scored Ponty four tries to one.
There were two other spectacular away wins. Aberavon won at Rodney Parade for the first time in 22 years, 19-6, while Swansea were 36-30 victors at Cardiff Arms Park.
Wizards outside-half Jamie Davies booted 14 points to steer his side to a first ever league win at Newport - his fourth penalty in the 75th minute winning the game - while All Whites' Stuart Thomas went one better against Cardiff with five penalties in a 17-point haul.
Maesteg pushed Llanelli all the way at Stradey Park before going down 17-16. The home side out-scored the visitors three tries to one but Maesteg outside-half Luke Richards scuffed a chance to win the game when his 83rd-minute penalty drifted wide.
The all-Gwent derby saw Bedwas beat Cross Keys 18-3 thanks to four penalties from centre Phil Dolman.