Lambert was crowned king in May 891 at Pavia[1] and joint emperor alongside his father on 30 April 892 at Ravenna by a reluctant Pope Formosus.

While Berengar occupied Pavia, Lambert and Ageltrude travelled to Rome to receive papal confirmation of his imperial title,[5] but Pope Formosus wanted instead to crown Arnulf and was imprisoned in the Castel Sant'Angelo.

Lambert was preoccupied in thwarting the attempts of both Arnulf of Carinthia and Berengar of Friuli to take Italy for themselves during his reign.

Despite the urging of Fulk of Rheims on his behalf, Lambert found himself abandoned by the Pope, who feared the increased power of the Spoletan house.

Finding Rome locked against him and held by Ageltrude, Arnulf took the city by force on 21 February 896, freeing the pope.

[9] The vengeful Lambert and Ageltrude also persuaded Pope Stephen VI, elected by their influence, to put the corpse of Formosus on trial for various crimes.

[citation needed] The body, stripped of its papal robes and mutilated, was thrown into the river Tiber after the "Cadaver Synod.

Seventy bishops met and confirmed the pact of 891, the invalidity of Arnulf's coronation, and the validity of Lambert's imperial title.

[11] Lambert hereafter governed with the church and continued the policy of his father of renovatio regni Francorum: renewal of the Frankish kingdom.

On his return to Marengo however, he was killed, either by assassination (by Hugh, son of Maginulf), a theory about which Liutprand, our primary source, is reserved, or by falling from his horse.

His epitaph (in Latin elegiac couplets) is: He was succeeded in Spoleto by Guy IV while the regnum Italicum and the imperium Romanum were thrown into chaos, contested by multiple candidates.