The agency, previously known as HouseSimple, raised eyebrows among the property industry in 2018 when it shifted to a no-fee model.
Its main form of revenue is by selling extras such as marketing boosts, hosted viewings and earning referrals for mortgage advice and arranging removals.
The model was initially trialled in Yorkshire and the north west of England but has gradually spread into southern England.
Three years on, Mitchell told said the landmark figure shows the model works.
He said: “We are delighted to reach this pivotal milestone — it not only is proof of how far we’ve come and what the whole company has accomplished, but it shows that what we are doing is working.
“Our innovative model continues to challenge the status quo and win market share as we continue to grow and expand further south.”