Ashfaq Parvez Kayani was born at Manghot, a village located in Punjab Province, on 20 April 1952. The town of Manghot is situated on the Pothohar Plateau bounded on the east by the Jhelum River, on the west by the Indus River. Ashfaq's Father was a Junior commissioned officer (JCO) in the Pakistan Army as Subedar major.

Ultimately, Kayani gained commissioned as 2nd Lieutenant in the 5th Battalion of the famed Baloch Regiment on 29 August 1971. He actively participated and joined up the military in time of 1971 war with India.

General Kayani is noted as the second four-star army general who held the directorship of the ISI but later then went on to become the Chief of Army Staff. The first appointment of ISI Director being appointed to four-star appointment was in 1999 when General Ziauddin Butt was the first army general who was appointed as army chief after his brief tenure as ISI Director.

As Lieutenant-Colonel, Kayani commanded an Infantry Battalion and an Infantry Brigade, as Brigadier. Later, he served in the government of former Prime Minister Benazir Bhutto as her deputy military secretary during her first stint as prime minister. Upon his promotion to two-star rank, Major-General Kayani served as the general officer commanding of the 12th Infantry Division stationed in Murree, deployed all over the LOC region and which comes under the X Corps. In 2000, Kayani was moved and appointed as the Director of the Directorate–General of Military Operations (DGMO). In 2001, it was during his tenure as DGMO that the intense military standoff between Pakistan and India took place. Reportedly, Kayani only slept a few hours a night during that period as he diligently oversaw the unified armed forces mobilisation and preparedness on the border.

During Kayani's tenure at the X Corps, he led the successful investigation of the two back-to-back suicide attacks against Musharraf in December 2003. It is believed that Kayani won the trust of Musharraf after the investigation, and a prelude to Kayani's appointment as the sensitive position of ISI chief. He was awarded Hilal-e-Imtiaz, the civilian medal, for his achievement.

In October 2004, Lieutenant-General Ashfaq Kayani was appointed as the Director general of Directorate of Inter-Services Intelligence (ISI), in place of General Ehsan-ul-Haq, who was promoted as the Chairman of the Joint Chiefs of Staff Committee. General Kayani directed the ISI operations and her operatives during a bleak period, with widely spread insurgencies in North-West Pakistan and Balochistan, disclosing of the nuclear proliferation case, and waves of suicide attacks throughout Pakistan emanating from the northwestern tribal belt. In his final days at the ISI, he also led the talks with Benazir Bhutto for a possible power sharing deal with Musharraf. In October 2007, after three years, he was replaced at the ISI by Lt Gen Nadeem Taj.

In October 2007, Kayani's promotion papers for the appointment to the four-star rank was approved by the President Musharraf, and appointed him as the Vice Chief of Army Staff (VCOAS). At the time of the promotion, Kayani ultimately superseded the senior-most army general, Lt-Gen. Khalid Kidwai, who was on an extension for one year. On 28 November 2007, Kiyani succeeded Musharraf as chief of army staff after Musharraf's retirement.

On 7 March 2008 General Kayani confirmed that Pakistan's armed forces will stay out of politics and support the new government. He told a gathering of military commanders in the garrison city of Rawalpindi that "the army fully stands behind the democratic process and is committed to playing its constitutional role." The comments made were after the results of the Pakistani general election, 2008 where the Pakistan Peoples Party won the election and began forming a coalition government who were opposed to President Pervez Musharraf.

Kayani's comments about the Datta Khel strike came to be put in the broader context of public and private communications by Pakistani officials with Washington, including an April 2011, visit by the head of the ISI, Lt. Gen. Ahmed Shuja Pasha, to CIA Director Leon Panetta at CIA headquarters. "[S]ome officials in both countries [were] saying intelligence ties [we]re at their lowest point since the Sept. 11, 2001, attacks spurred the alliance," according to one report. The report went on to say the overall communications included private demands that the CIA suspend drone strikes and also reduce the number of US intelligence and Special Operations personnel in the country. After the ISI-CIA meeting, CIA spokesman George Little said the intelligence relationship "remains on solid footing."

His retirement was eventually confirmed when Prime Minister Nawaz Sharif approved then-Lieutenant General Raheel Sharif as chief of army staff and Lieutenant General Rashid Mehmood as Chairman Joint chiefs on November 27, 2013.