The pivotal moment of the clash came when, just seconds into the second spell, a chargedown led to a try to St Kentigern No 10 and official man of the match Rivez Reihana.
Twice MAGS turned down shots at goal, but their passing in the backline was not fluent. St Kentigern had less ball but marked up on defence to consign MAGS to their first defeat of 2016.
In the semifinals this Saturday, MAGS will host St Peter's, who beat Kelston BHS 23-7. A fine solo try to No 6 Callum McNab was the highlight of St Peter's display, which sealed fourth place with a 9-1-1 record.
Sacred Heart were dismayed at the MAGS-St Kentigern result, as they had put together a solid, bonus point, 24-0 shutout of Auckland Grammar, which saw them regain the Henry Cooper-Brother Maurice Memorial Trophy. A win to MAGS would have seen Sacred Heart's first home semifinal since 2011. Instead, they will again travel to St Kentigern for an extraordinary fourth semi in five seasons.
Centre Lemeki Namoa recorded a hat-trick and wing Melino Huihui'uiha scored a tremendous solo try, but the star turn was No 8 Hoskins Sotutu, whose aerial and attacking skills were on full show.
King's, mentally spent after MAGS extinguished their semifinal hopes 10 days ago, were edged 10-7 by south Auckland rivals De La Salle, for whom centre Lalomilo Lalomilo scored a try and set one up.
Aorere ran in some nice tries on a heavy field to blank Otahuhu 41-0 and clinch the Keven Mealamu Trophy.
Dilworth won their third game of the 2016 1A, 19-7 over Onehunga, with a brilliant solo try from more than 50m to first five Moli Faiva the clear highlight.
Onehunga and Otahuhu will again contest the promotion-relegation series with the yet to be decided 1B finalists in April 2017.
In the North Harbour 1A competition, Whangarei BHS have cracked the top four at their first attempt after dispatching Birkenhead 46-0. They will play Westlake BHS, who qualified top after beating Takapuna Grammar 54-15, while Rosmini had second spot sewn up after beating Orewa 43-3. Rosmini will host Massey in the other semifinal, after the westerners defeated Rangitoto 15-3.
The Super 8 final on Saturday will pit unbeaten Hastings BHS against defending champions and perennial heavyweights Hamilton BHS.
They were supposed to meet in the final round-robin match last Saturday in Hastings, but the extreme weather in Hawke's Bay saw the clash abandoned. It mattered not for the table, as both sides were well clear in first and second position.
Rotorua BHS placed third, but were edged 25-24 by New Plymouth BHS. A late try to centre Jack Stratton saw Palmerston North snatch a 17-all draw in their Polson Banner clash at Napier BHS. The visitors did well to claw back from a 14-0 deficit.
*The August 20 Central North Island final looks set to be St Paul's Collegiate against St Peter's of Cambridge in an all-Waikato decider.
In chilly conditions, tries to Sean Chenery, Jake Hunter and Jacob Norris saw St Peter's to a narrow 17-16 victory at Wanganui Collegiate. St Paul's Collegiate rolled St John's of Hastings 52-7.
Sunday's Wellington final will see defending champion St Pat's Town take on Wellington College after both sides won their semifinals on the weekend.
The Cup semifinalists have been found in the UC Championship in the Crusaders' region.
Christ's College, who hammered STAC 69-17, will host Shirley BHS, who went down 19-13 to second qualifiers Christchurch BHS. The latter will play Nelson College, who in turn defended the Moascar Cup with a heavy 55-5 defeat of Lincoln HS.
Otago honours went to John McGlashan, 23-0 victors over Waitaki BHS in the final.
North Harbour 1A:
Rosmini 43 Orewa 3
Whangarei BHS 46 Birkenhead 0
Westlake BHS 54 Takapuna Grammar 15
Massey 15 Rangitoto 3
Auckland 1A:
St Peter's 23 Kelston BHS 7
Dilworth 19 Onehunga 7
Sacred Heart 24 Auckland Grammar 0
Aorere 41 Otahuhu 0
De La Salle 10 King's 7
St Kentigern 8 MAGS 3
Auckland 1B Championship:
Tamaki 20 Tangaroa 5
Liston 29 Botany Downs 3
One Tree Hill 10 Mangere 0
Waitakere 22 Macleans 5
Auckland 1B Plate:
Southern Cross 53 Pakuranga 16
Mt Roskill Grammar 29 Edgewater 11
Howick 17 Papatoetoe 5
Avondale 12 St Paul's 0
Counties Manukau/Waikato:
St John's (Hamilton) 70 Matamata 0
Pukekohe beat Alfriston by default
Manurewa 38 Cambridge 10
Super 8:
Hastings BHS v Hamilton BHS cancelled due to extreme weather
Tauranga BC 22 Gisborne BHS 20
New Plymouth BHS 25 Rotorua BHS 24
Napier BHS 17 Palmerston North BHS 17 (Polson Banner)
Central North Island:
Feilding 14 Wesley 7
St Peter's (Cambridge) 17 Wanganui Collegiate 16
St Paul's Collegiate 52 St John's (Hastings) 7
Wellington semifinals:
St Patrick's (Town) 21 Rongotai 3
Wellington 29 Wairarapa 0
UC Championship:
Christ's 69 St Andrew's 17
Timaru BHS 15 St Thomas' 10
Christchurch BHS 19 Shirley BHS 13
Ashburton 26 Roncalli 24
Nelson 55 Lincoln 5 (Moascar Cup)
Marlborough BC 16 St Bede's 10
Waimea Combined 52 Rangiora 12
Otago final:
John McGlashan 23 Waitaki BHS 0