He turned British Airways into IAG.
But now Willie Walsh is handing over the controls.
He will be replaced by Luis Gallego, currently head of group firm Iberia.
Walsh has been head of IAG since 2011.
He oversaw the initial merger of British Airways and Iberia.
And its subsequent acquisition of other carriers including Aer Lingus and Vueling.
Now the company says he will step down in late March, before retiring fully in June.
Walsh calls Gallego the right man to take over.
But he leaves some issues for his successor to address.
A December survey by consumer group Which rated British Airways as the second-worst long-haul airline.
The carrier's reputation has also been tarnished by labour unrest and computer failures that left customers stranded.
IAG shares were up over 1% in early trade Thursday (January 9).