Sharing the same design cues of the smaller single-deck Alexander ALX200 and double-deck Alexander ALX400, the ALX300 has a rounded roof dome similar to the ALX200, a deep double-curvature windscreen with plastic mouldings under the windscreen to make it look deeper and a separate destination sign.

The first 23 Alexander ALX300s entered service with Stagecoach Busways from early 1998 onwards, all of which were built on the Volvo B10BLE chassis.

[1] Stagecoach soon chose to standardise on ALX300s on the MAN 18.220 chassis by ordering 150 of the type following the Coach & Bus '97 expo,[2] taking delivery of its first 28 for its Thames Transit subsidiary's Oxford operations from November 1998 onwards,[3] followed by major batch orders for its Manchester, Busways, Transit, Cumberland, Fife Scottish and Bluebird subsidiaries.

Orders for ALX300s on the MAN chassis continued following the group's rebranding in November 2000, despite the arrival of the Alexander Dennis Enviro300 in 2001, which was aimed at the ALX300's market.

The first ALX300s ordered following this rebrand were delivered to Stagecoach East in late 2001, with 40 ALX300s on MAN 18.220 chassis being used to relaunch its Cambridge bus network under the 'citi' brand.