Einhard was from the eastern German-speaking part of the Frankish Kingdom. Born into a family of relatively low status, his parents sent him to be educated by the monks of Fulda - one of the most impressive centres of learning in the Frankish lands - perhaps due to his small stature (Einhard referred to himself as a "tiny manlet") which restricted his riding and sword-fighting ability, Einhard concentrated his energies towards scholarship and especially to the mastering of Latin. Despite such humble origins, he was accepted into the hugely wealthy court of Charlemagne around 791 or 792. Charlemagne actively sought to amass scholarly men around him and established a royal school led by the Northumbrian scholar Alcuin. Einhard evidently was a talented builder and construction manager, because Charlemagne put him in charge of the completion of several palace complexes including Aachen and Ingelheim. Despite the fact that Einhard was on intimate terms with Charlemagne, he never achieved office in his reign. In 814, on Charlemagne's death his son Louis the Pious made Einhard his private secretary. Einhard retired from court during the time of the disputes between Louis and his sons in the spring of 830.

Einhard was married to Emma, of whom (as of most laywomen of the period) little is known. There is a possibility that their marriage bore a son, Vussin. Their marriage also appears to have been exceptionally liberal for the period, with Emma being as active as Einhard, if not more so, in the handling of their property. It is said that in the later years of their marriage Emma and Einhard abstained from sexual relations, choosing instead to focus their attentions on their many religious commitments. Though he was undoubtedly devoted to her, Einhard wrote nothing of his wife until after her death on 13 December 835, when he wrote to a friend that he was reminded of her loss in ‘every day, in every action, in every undertaking, in all the administration of the house and household, in everything needing to be decided upon and sorted out in my religious and earthly responsibilities’.

Einhard made numerous references to himself as a "sinner", a description of himself that shows his Augustinian influenced world view. To assuage such feelings of guilt he erected churches at both of his estates in Michelstadt and Mulinheim. In Michelstadt he also saw fit to build a basilica completed in 827 and then sent a servant, Ratleic, to Rome with an end to find relics for the new building. Once in Rome, Ratleic robbed a catacomb of the bones of the Martyrs Marcellinus and Peter and had them translated to Michelstadt. Once there, the relics made it known they were unhappy with their new tomb and thus had to be moved again to Mulinheim. Once established there, they proved to be miracle workers. Although unsure as to why these saints should choose such a "sinner" as their patron, Einhard nonetheless set about ensuring they continued to receive a resting place fitting of their honour. It has been contended that in the last decade of his life Einhard's strong religious beliefs led to him retiring to a monastery. However, his letters from this period show his maintained contact with those he had met in court and Julia Smith has claimed the tone of these letters are not as religious in character as would have been expected from a member of the church. After his death he was buried in Mulinheim (today Seligenstadt) with his wife Emma, near his beloved saints Peter and Marcellinus.

Local lore from Seligenstadt portrays Einhard as the lover of Emma, one of Charlemagne's daughters, and has the couple elope from court. Charlemagne found them at Seligenstadt (then called Obermühlheim) and forgave them. This account is used to explain the name "Seligenstadt" by folk etymology. Einhard and his wife were originally buried in one sarcophagus in the choir of the church in Seligenstadt, but in 1810 the sarcophagus was presented by the Grand Duke of Hesse to the count of Erbach, who claims descent from Einhard as the husband of Imma, the reputed daughter of Charlemagne. The count put it in the famous chapel of his castle at Erbach in the Odenwald.

The most famous of Einhard's works was produced at the request of Charlemagne's son and successor Louis the Pious, his biography of Charlemagne, the Vita Karoli Magni, "The Life of Charlemagne" (c. 817–836), which provides much direct information about Charlemagne's life and character. In composing this he relied heavily upon the Annals of the Frankish Kingdom. Einhard's literary model was the classical work of the Roman historian Suetonius, the Lives of the Caesars. His work was written as a praise of Charlemagne, whom he regarded as a foster-father and to whom he was a debtor "in life and death". The work thus contains an understandable degree of bias, Einhard taking care to exculpate Charlemagne in some matters, not mention others, and to gloss over certain issues which would be of embarrassment to Charlemagne, such as the morality of his daughters.

He is also responsible for three other extant works: a collection of letters, On the Translations and the Miracles of SS. Marcellinus and Petrus, and On the Adoration of the Cross. The latter dates from ca. 830 and was not rediscovered until 1885, when Ernst Dümmler identified a text in a manuscript in Vienna as the missing Libellus de adoranda cruce, which Einhard had dedicated to his pupil Lupus Servatus.