In the 12th and 13th centuries, during the Crusader period, Ajjul was inhabited by Muslims, according to the Muslim scholar Diya al-Din (d. 1245). A mosque in the village has an inscription in the south wall, dating it to 1196. The inscription is in Ayyubidnaskhi script.
Röhricht (1842–1905 suggested that Ajjul was the Crusader place called Gul; however, Conder (1848–1910) disagreed.
The village was incorporated into the Ottoman Empire in 1517 with all of Palestine, and in 1596 it appeared in the tax registers as being in the Nahiya of Quds of the Liwa of Quds. It had a population of 79 households, all Muslim. They paid a fixed tax rate of 33,3% on agricultural products, which included wheat, barley, summer crops, vineyards and fruit trees, olives, goats and/or beehives; a total of 8,745 akçe. Half of the revenue went to a Waqf.
In 1838 Ajjul was noted as a Muslim village in the Beni Zeid administrative region. In 1870 Victor Guérin passed by the village, which he called A'djoul, and estimated it to have about 300 inhabitants. Around Ajjul he found large fig and carob-trees, besides pomegranate, mulberry and apricot trees. An official Ottoman village list from about the same year showed that Ajjul had 79 houses and a population of 250, though the population count included men, only.
In 1882, the PEF's Survey of Western Palestine described Ajjul as a "village of moderate size, with a well. It is on high ground, with olives round it, and ancient tombs. An ancient road leads towards it on the south." In 1896 the population of Ajjul was estimated to be about 468 persons.
In the 1945 statistics, the population was 350 Muslims, and the total land area was 6,639 dunams, according to an official land and population survey. Of this, 3,507 were allocated for plantations and irrigable land, 863 for cereals, while 14 dunams were classified as built-up areas.
After the 1995 accords, 48.3% of Ajjul land is defined as Area A land, 27.2% is Area B, while the remaining 24.5% is defined as Area C. Israel has confiscated 363 dunams of village land in order to construct the Israeli settlement of Ateret.
In a 1997 PCBS census, 4.2% of Ajjul's population — which was 1,026 — were Palestinian refugees. The largest age group in the village were infants to 14-year-olds, making up 44.2% of the population. About 25.3% of the population is between the ages of 15 and 29, 24.2% between 30 and 64 and residents 65 or older represent 6.3% of the population. There were slightly more males (51.7%) than females (49.3%) in Ajjul's gender make-up. In the 2007 PCBS census, the figures of Ajjul's population showed a smaller population of 1,237 people, of which 601 were males and 636 were females.
Ajjul contains a clinic that is primarily involved in blood testing. Most of the residents receive medical help from the Palestinian Red Crescent stationed in nearby Sinjil. The nearest hospital is in Ramallah.
Two mosques are located in Ajjul: a modern one and an older renovated one.
There is a mixed-gender secondary school in the village, in which 400 students are enrolled. Students attend science and literature classes at the Prince Hassan School in Bir Zeit. Ajjul has about 50 college and university students. There is no postal service in the village.