The speed of the deal was perhaps the biggest shock, with Aston Martin wasting no time in replacing one of Formula One's greats, four-time world champion Sebastian Vettel (35), with another.
Vettel had announced his retirement at the end of the season last Thursday, triggering immediate speculation about his replacement.
Double world champion Alonso figured highly on that list, given that the Spaniard had yet to agree a deal to continue with Alpine despite being the oldest driver on the grid (turning 41 last Friday).
Alpine have young Australian reserve Oscar Piastri (21), a driver who has won a string of junior titles at the first attempt including last year's Formula Two championship, knocking on the door.
Team boss Laurent Rossi had spoken last month of possibly loaning Piastri, who is highly rated but still with much to learn, to another team for a year.
Alonso, he indicated, could then be switched to the Alpine endurance effort.
The Spaniard wanted a longer-term deal, however, and was unlikely to accept a secondary stand-in role essentially keeping a seat warm for someone else after a season in which he has kept teammate Esteban Ocon (25) on his toes.
Aston Martin's quick swoop for Alsonso indicates that this is a team that means business even if they are still a long way from the podium.
"The recruitment of a special talent such as Fernando is a clear statement of intent from an organisation that has committed to developing a winning Formula One team," Aston said.
With Aston Martin, Alonso gets to stay at the pinnacle of the sport and set more records for longevity - most laps and most races - if not wins and titles as well.
Canadian teammate Lance Stroll (23), son of the team's billionaire owner Lawrence, is secure and has little to fear.
As for Alpine, Alonso's departure is easy to respond to with Piastri now the obvious choice to partner with the incumbent Frenchman Ocon.
Formula One, a sport increasingly dominated by young drivers, meanwhile gets to keep one of its established stars after the departures of Vettel and, last year, 2007 world champion Kimi Raikkonen (42).
