Scarlets moved into the top four of the PRO12 standings after their impressive 38-29 victory over current table toppers Leinster on Friday.
Johnny McNicholl, Werner Kruger and Steffan Evans all scored first-half tries, matched only by Leinster's Richardt Strauss, before Barry Daly was shown a straight red card for a dangerous tackle early in the second half.
Scarlets extended their lead through Will Boyde and Ryan Elias, before Leinster mounted a comeback with tries from Adam Byrne, Tom Daly and Andrew Porter.
With seven minutes gone debutant McNicholl was the first to carry over, as the full-back latched on to a loose ball and scored from 40 metres to allow Patchell to open his account from the tee.
After Mike McCarthy was sent to the sin-bin, the hosts soon made the extra man pay as Kruger scored from the back of a driving maul, and Patchell slotted his second kick of the evening from wide on the left.
Isa Nacewa opened the Irish club's account from the tee, before Evans battled his way over the whitewash to make the most of an extended spell of Scarlets possession, and Patchell added the extras.
Leinster hit back minutes before the break when Adam Byrne broke down the right-hand side, before finding hooker Strauss inside him to score an unconverted try. The visitors dismal evening only got worse when winger Daly was shown a straight red card for tackling Aled Thomas in the air, who landed dangerously.
Number eight Boyde then carried over in the corner to secure a try-scoring bonus point, finishing off a great display of quick passing through the Scarlets backs before Patchell added the touchline conversion.
The hosts then looked to work another lineout drive, and although Leinster halted it short of the line the ball made its way in to the hands of lock Elias, who barraged his way over with the help of Aaron Shingler, as Patchell maintained his 100 percent record from the tee.
Despite being down to 14 men Leinster went on the hunt for a bonus point, and soon had their second try of the night as winger Byrne's quick feet saw him dance through a lacklustre Scarlets defence, before Nacewa added two more points from the tee.
The visitors comeback continued as replacement Daly pounced on Ross Byrne's clever grubber kick under the posts, making Nacewa's conversion a formality, and the score 35-22.
Dan Jones 71st minute penalty halted the visitors' scoring streak, but Porter's late score meant Leinster returned home having secured a losing bonus point.
Tries: Mcnicholl, Kruger, Evans, Boyde, Elias
Cons: Patchell 5
Tries: Strauss, Byrne, Daly, Porter
Cons: Nacewa 3
Red Card: Barry Daly (41st minute)
Scarlets: 15 Aled Thomas, 14 Johnny Mcnicholl, 13 Steff Hughes, 12 Hadleigh Parkes (c)], 11 Steff Evans, 10 Rhys Patchell, 9 Jonathan Evans, 8 Will Boyde, 7 James Davies, 6 Aaron Shingler, 5 David Bulbring, 4 Tom Price, 3 Werner Kruger, 2 Ryan Elias, 1 Wyn Jones
Replacements: 16 Emyr Phillips, 17 Dylan Evans, 18 Peter Edwards, 19 Tadhg Beirne, 20 Josh Macleod, 21 Aled Davies, 22 Dan Jones, 23 Gareth Owen
Leinster: 15 Isa Nacewa (c), 14 Adam Byrne, 13 Rory O’Loughlin, 12. Noel Reid, 11 Barry Daly, 10 Ross Byrne, 9 Jamison Gibson-Park, 8 Jack Conan, 7 Dan Leavy, 6 Dominic Ryan, 5 Ian Nagle, 4 Mike McCarthy, 3 Michael Bent, 2 Richardt Strauss, 1 Peter Dooley
Replacements: 16 James Tracy, 17 Andrew Porter, 18 Oisín Heffernan, 19 Ross Molony, 20 Peadar Timmins, 21 Luke McGrath, 22 Tom Daly, 23 Zane Kirchner
Referee: Sean Gallagher (Ireland)
Assistant Referees: Leo Colgan (Ireland), Elgan Williams (Wales)