The models are being fitted with a belt-driven starter generator and 48 volt on-board power supply on the 2.0-litre 35 4Matic versions.
Power is now up to 305bhp with the 0 to 62mph acceleration time for the GLA now 5.2 seconds with the larger GLB achieving the sprint in 5.5 seconds.
Top speed for both is just in excess of 155mph.
Fuel economy is now 26.7mpg for the GLA and 26.4mpg for the GLB, with respective emissions of 200 and 203g/km.
As well as the new engine system, the cars also get a new front apron design, LED lights front and rear and an AMG Performance steering wheel as standard.
Pricing for the mild-hybrid models is still to be announced, but the current versions are priced from £53,995 for the GLA 35 and £53,295 for the GLB 35.
Prices correct as of 16/03/2023 and are subject to change.