Diego Simeone has been at Atletico since December 2011, for 14 years. Two La Liga titles, high-level status and a recognizable style - this is an era.
According to Lent.az, citing foreign media, but even this is not enough to take first place.
The longest-serving coach in Europe's top five leagues is Frank Schmidt.

He has been in charge of Heidenheim since 2007, leading the club from the third league to the Bundesliga.
Top 5 most stable coaches:
• Frank Schmidt ("Heidenheim") - since 2007
• Diego Simeone ("Atletico") - since 2011
• Pep Guardiola ("Manchester City") - 9+ years
• Mikel Arteta ("Arsenal") - 6 years
• Manuel Pellegrini ("Betis") - 5+ years