In the girls section, St Hilda's Collegiate finished third.
McGlashan won a tense virtual final in the last round-robin tie beating Christ's College by one set on countback, after rubbers were tied at 3-3.
In earlier rounds, it also accounted for Nelson College 6-0, Timaru Boys' 5-1 and St Bede's 5-1.
It lost by three games on countback against Christchurch Boys' High but, with other results favouring it, was able to take the overall title.
The individual stars were Carlos Reid, playing at No 2, and Mitchell Sizemore at No 3.
Both boys have suffered from serious illness within the past year, but bounced back with unbeaten singles records at the event.
McGlashan was able to cover a lack of depth in the lower order by careful placement of its doubles pairings, along with Reid and Sizemore's results.
The other members were Paddy Ou, playing in the difficult top spot, along with Tim James and Lucas Reid, who shared the bottom singles position and played a part in doubles successes.
St Hilda's beat Rangi Ruru 6-0, Central Southland 5-1 and Ashburton College 5-1, but went down to St Margaret's 6-0 and Christchurch Giirls' High 4-2.
The team members were Jessie Stevenson, who had a series of long, close tussles at No 1, Meg Timu, Olivia Ray, Olive Duffy and Katie Duncan. Several of the girls will be eligible for several years to come, giving hopes for an even higher finish in future.
