The result was only Mary's second win in their last AIB League six games and soon Sexton was making his mark for Leinster.
The 23-year-old and his big right boot will be part of Leinster's matchday squad for this Saturday's European encounter with Wasps.
But Mary's, currently top of the Division One table after two wins out of two, have other potential match winners to call upon for the visit of Clontarf on Saturday afternoon.
It will be first plays fifth at Castle Avenue as two of the clubs that are leading Leinster's current resurgence in the AIB League's top flight go head-to-head.
Two bonus point victories speak for themselves, and with backs coach Shaun McCarthy, an astute summer signing from Wicklow, and Matt D'Arcy starring in the league's form half-back partnership, Mary's are enjoying a purple patch.
The brothers Sweeney, Rob and Richie, are making their mark, as are second row duo Gareth Logan and Conor McInerney and combatitive flanker Paul Nash.
Behind the scrum, McCarthy, D'Arcy, Stephen Grissing and former Connacht utility back Conor McPhillips have all helped themselves to tries in the last fortnight.
How the Clontarf back-line measures up to the Mary's one will decide much as the sides lock horns in the AIB League for the first time since last November's meeting at Templeville Road.
Back then, 'Tarf emerged as 31-15 winners with then half-backs Ian Keatley and Paul O'Donohoe running the show. O'Donohoe remains but Keatley has quickly become a Connacht hero this season.
The O'Donohoe v D'Arcy battle at number 9 will be fascinating to watch. Both players have experience of the Leinster Academy (D'Arcy in previous years and O'Donohoe is a current member), they possess a strong pass off either side and are attack-minded.
While Mary's, under new coach Ciaran Potts, are hogging the headlines at the top of the table, 'Tarf are quietly going about their business under Andy Wood.
The north Dubliners have made their best start to an AIB League campaign since 2004 and have made Castle Avenue something of a fortress.
Granted, they are not playing as well as they can at the moment and their back-line has yet to click into gear, but in South African flanker Heinrich Stride they boast one of the signings of the season.
Stride really imposed himself in 'Tarf's opening league win over Garryowen and with both power and pace and a keen eye for a big hit, the Currie Cup-capped player can have quite an impact on a game.
One player who it is heartening to see back near his best is Clontarf centre Breffni O'Donnell.
The AIB Club international player has struggled with injuries in recent seasons but he showed his class in setting up an excellent try for league debutant Michael Keating last weekend.
'Tarf are coming off the back of a tense 19-16 win over Terenure College, with a late try from teenager Keating seeing them take the spoils.
Keating, Max Rantz McDonald and Niall O'Brien are good finishers and will be hoping to put away the chances created by the likes of O'Donnell, Daragh O'Shea and out-half Martin Dufficy.
'Tarf lined out with a formidable back row unit of Stride, Cameron Jowitt and Simon Crawford last Saturday and if they are retained, Mary's could have their work could out there.
However, the Templeville Road residents have the revenge factor to consider as they lost 9-6 to 'Tarf in last season's Leinster Senior Cup final and another such defeat could ruin their early good work.
Meanwhile, at Stradbrook on Saturday, Terenure College will pay Blackrock College a visit in what has the makings to be another lively Dublin derby.
Kevin West's 'Nure outfit were better than their 43-19 opening day defeat to Cork Constitution suggests and luck was also not on their side when they were edged out by Clontarf last weekend.
Third time lucky? Possibly, but they will have to go some to beat a Blackrock team that impressively achieved back-to-back wins over Shannon and Garryowen.
In centre-cum-out-half David McAllister, Terenure have a match winner and someone who can unlock defences with the deftest of passes or the most accurate of kicks.
Mark Mahony, David Cazabon and John Bollard are strong presences behind what is a sizeable scrum. The 'Nure eight, led by captain Alex Dunlop, included Leinster's Brian Blaney and Stephen Knoop last weekend.
But it is hard to look past a third successive Blackrock win, as all over the pitch this season, they seem to have quality players who play with cleverness and confidence.
Coaches Barry Gibney and Emmet Farrell have a mix of youth and experience at their disposal and a number of well-drilled, new players have pushed through, not least out-half Darragh Fitzpatrick.
The youngster already has 33 points to his name from two games and his coolness under pressure stems from the guidance that Shane Byrne, the captain this season, and Des Dillon give the side throughout the 80 minutes.
Former Ireland Under-20 winger Shane Monahan has notably opened the league campaign with four excellent tries, centres Simon Morrissey and Kyle Tonetti are an all-action pairing and full-back Niall Morris and scrum half David Moore both have bright futures.
The Blackrock pack is not too shabby either and they graft away, creating the platforms from which this talent-laden back-line strike. If they can cut loose in the second half, a home win is assured.
The third Dublin derby in the league this weekend will be staged at Donnybrook where Bective Rangers entertain an Lansdowne side.
The two Dublin 4 clubs, both currently in the top six of the Division Two table, have started the season well.
Bective put 36 points on DLSP last Sunday for their first win, while Lansdowne, after a thrilling 19-19 draw with champions UCC, stormed past Instonians on a 37-6 score-line at the RDS last weekend.
Another points blitz from their influential, new out-half Charl Oosthuizen should see Lansdowne first past the winning post.
2008/09 AIB LEAGUE: Saturday, October 18
Kick-off 2.30pm unless stated -
Shannon v Garryowen, Thomond Park Stadium (Friday, October 17, 7.30pm)
Ballymena v Galwegians, Eaton Park
Blackrock College v Terenure College, Stradbrook
Buccaneers v UL Bohemians, Dubarry Park
Clontarf v St. Mary's College, Castle Avenue
Cork Constitution v Young Munster, Temple Hill
Dungannon v UCD, Stevenson Park
Old Belvedere v Dolphin, Anglesea Road
Bective Rangers v Lansdowne, Donnybrook
Bruff v Belfast Harlequins, Kilballyowen Park
Clonakilty v Instonians, Shannonvale Cross
De La Salle Palmerston v Highfield, Kilternan
Greystones v Dublin University, Dr. Hickey Park
Thomond v Malone, Liam Fitzgerald Park
UCC v Old Crescent, The Mardyke
Wanderers v Ballynahinch, Merrion Road
Banbridge v Sundays Well, Rifle Park
County Carlow v Ards, Oak Park
Connemara v Suttonians, Monastery Field
Corinthians v Rainey Old Boys, Corinthian Park
Midleton v Old Wesley, Towns Park
Naas v Nenagh Ormond, Forenaughts
Portadown v Barnhall, Chambers Park
Queen's University v Waterpark, Dub Lane
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