The former Tottenham Hotspur man signed a new deal over the summer to keep him at the club for another season, although he is no longer a weekly starter under Carlo Ancelotti.
Nevertheless, Modric had taken advantage of injuries to fellow midfielders to start four of Real's five La Liga matches before the international break, providing an assist in their 2-0 success over Villarreal.
The Croatia international was dropped to the bench for this weekend's trip to Celta Vigo, though, as Federico Valverde, Aurelien Tchouameni, Eduardo Camavinga and Jude Bellingham occupied the central positions.
Kylian Mbappe's 20th-minute strike put Ancelotti's men ahead, but Celta Vigo levelled early in the second half through Williot Swedberg, after which the Real boss brought Modric into the fray.
Modric becomes oldest player in Real Madrid history
At 39 years and 40 days old, Modric became the oldest player to ever make an appearance for the 15-time European champions, breaking the previous record set by Ferenc Puskas in 1966.
Modric was four days older than Puskas when he played his final match for Los Blancos at 39 years and 36 days, and he provided the assist for Vinicius Junior's winner within three minutes of coming on.
The midfielder's telling contribution also ensured that he would hit another landmark on Saturday evening, as he brought up 250 wins in the top flight of Spanish football with Real Madrid.
Los Blancos' slender victory over Celta Vigo saw them go level on points with leaders Barcelona at the top of the La Liga table, although the Catalonian powerhouses will have the chance to respond against Sevilla on Sunday.
Modric, meanwhile, has now provided two assists in 12 appearances for Ancelotti's side during the current campaign, also setting up Antonio Rudiger's goal in September's 3-1 Champions League win over Stuttgart.
Modric's Real Madrid achievements: A breakdown
Becoming the oldest player in Real Madrid's history is simply an added bonus to Modric's extraordinary CV, as he is already the most successful player in the history of the Real Madrid men's team.
Indeed, the 39-year-old has won a staggering 27 major honours during his time in the Spanish capital; six Champions League titles, four La Liga titles, five Club World Cups, five European Super Cups, five Spanish Super Cups and two Copa del Reys.
All in all, Modric has scored 39 goals and set up 90 more in 548 matches for Real Madrid, and he is one of eight men to have won the Ballon d'Or while representing the capital giants.
Furthermore, only Cristiano Ronaldo has also won The Best FIFA Men's Player award as a Real Madrid player, although whether he earns another contract extension to continue playing into his 40s remains to be seen.