More than 120 people graced Woodvale Reception Centre for the event on August 7, including founding priest the Rev Mark McCracken.
Event MC Bruce Herriman said St Anselm's began as an offshoot of St John's Anglican Church, Greenwood in 1993, with a small group of parishioners meeting at Kingsley's Creaney Primary School.
Two years later the rectory and worship centre opened nearby on Forest Hill Drive and was consecrated by the then Archbishop of Perth, Peter Carnley.
It wasn't until 2013 that the parish was able to employ its first full-time priest, the Rev Gillian Rookyard.
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Mr Herriman said the regular worshipping community at St Anselm's had grown steadily in the past two years and there was a great sense of optimism and encouragement about the years ahead.
As part of the 20-year celebrations, next month Archbishop Roger Herft and the new Warden of Wollaston College, Gregory Seach, will discuss the life and spirituality of the parish's patron saint, Anselm. The seminar on September 9 is open to all and begins at 7pm.