The Chilean forward was named among the substitutes by coach Igor Tudor for the game in Brittany, four days after signing a one-year contract.
He was introduced at the start of the second half but did not have a major influence on the game as Marseille failed to hold onto the lead given to them in the first half at the Stade Francis-Le Ble by Nuno Tavares.
Portuguese left-back Tavares scored his second goal in as many games since joining Marseille on loan from Arsenal to put his new side ahead shortly before half-time.
He converted a cross from Jonathan Clauss, only for Pierre Lees-Melou to equalise for Brest with a superb strike just after the hour mark.
Former Barcelona and Arsenal star Sanchez, who has won 143 caps for Chile, joined Marseille as a free agent after terminating his contract with Inter Milan.
He started just nine games for Inter last season, but Marseille are hoping his experience will prove invaluable in the Champions League group stage.
Tudor’s side have four points from a possible six in Ligue 1, with Sunday’s draw following a 4-1 win over Reims last week.
Reigning champions Paris Saint-Germain are already two points clear at the top of the table after just two games.
They followed a 5-0 win at Clermont on the opening weekend by beating Montpellier 5-2 on Saturday, with Neymar scoring twice and Kylian Mbappe once.