Replacements used: Darren Hudson for Lilly (half-time), Arthur Lyons for McGovern (45 mins), McGovern for McMahon (75), Lilly for Hogan (80).
Replacements used: Gearoid McDonald for Deasy (31 mins), Deasy for Kiersey (47), Ian Burke for Horan (55), Warren Larkin for Moran (71), Brian O'Callaghan for Boucher (74), Adam Boland for O'Reilly (75).
But the visitors quickly responded with a well-worked try by centre Mark Roche following some patient build-up play on the left wing by the Dublin 4 club.
This score enabled Lansdowne to settle into the proceedings and by the ninth minute, they had added their second try of the evening.
An enterprising maul underneath the St. Mary's posts was finished off by powerful hooker Tyrone Moran, and with full-back Scott Deasy adding a routine conversion Mike Ruddock's charges had shown that they meant business.
However, the Templeogue outfit managed to force their way back into the reckoning a couple of minutes later, as the lively Mark Sexton - a brother of Ireland international Jonathan - finished off a fine move across the opposition line in the left corner.
Leinster 'A' star Cathal Marsh was narrowly wide of the mark with his conversion attempt, but St. Mary's were unfazed by this missed opportunity.
Another terrific team move led to winger Chris Lilly crossing for another unconverted try on 17 minutes which gave the hosts the lead for the only time in the game.
St. Mary's dominated possession as the first half wore on, but they were not able to capitalise on the chances that fell their way. As a result, a brace of Tom Kiersey penalties in the 28th and 31st minutes helped Lansdowne to move back into the ascendancy.
This proved to be the catalyst for Lansdowne to end the half in impressive fashion, and when scrum half Paddy O'Driscoll secured a third try moments before the interval - converted with ease by his half-back partner Kiersey - they had built a 25-13 platform for the start of the second period.
In comparison to an incident-packed first half, the action upon the resumption was relatively subdued, although St. Mary's did reduce their deficit when Marsh expertly converted a 49th minute close range penalty.
There was never a sense that Lansdowne were going to let their lead slip away, however, and Marsh's kick was eventually cancelled out by a similar effort by former Cork Constitution star Deasy on 64 minutes.
The decisive moment of the game then arrived seven minutes later, as the excellent Moran bravely drove over the line for his second try and Lansdowne's bonus point score - opening up a 17-point margin in the process.
To the home side's credit, they battled until the bitter end and gave themselves a glimmer of hope in the closing moments when Sexton and flanker Gareth Austen both crossed the whitewash.
The late spurt saw them claim two bonus points, which may prove crucial at the end of the current campaign, but it was not enough to deny Lansdowne a well-deserved triumph.
Referee: Sean Gallagher (IRFU)