His son Arif told Geo News that his father died of breathing complications at an individual nursing home in Karachi.
The ghazal king were discharged from hospital only last month. He was back in hospital just a day later but was not placed on a ventilator.
"My father was ill for the last 12 years but this year, his condition has deteriorated significantly... within the last one month," said Arif. "He was released from hospital for only one day before being admitted again."
Hassan, who has an enormous fan following all around the world, have been affected by lung, chest and urinary tract ailments for the past many years.
Hassan leaves behind his nine sons and five daughters. He married twice. Both his wives are dead.
He first mesmerised connoisseurs along with his rendition of "Gulon main rang bhare", the first step of a protracted musical journey that lasted five decades. The death of the artist, who gave voice to ghazals similar to "Patta patta boota boota" and "Abke bicchde khwaabon mein mile", brings the curtains down on an era of lyricism, melody and poetry in light Hindustani classical music.
Born in undivided India in 1927, the ghazal singer, who kicked the bucket in a Karachi hospital Wednesday after a chronic illness, was instrumental in opening up the evocative world of Urdu poetry. He popularised the poetry form as a musical genre for millions of Pakistani and Indian homes within the process.
The soft-spoken man was the uncrowned king of music for Pakistan's movie industry and additional raised the profile of the ghazal, once considered passe among elite exponents of Hindustani classical music.
According to an estimate by his son Arif, Hassan gave voice to greater than 20,000 songs and, aside from Urdu, also sang in Bengali, Punjabi and Pashto.
Some of his famous ghazal renditions include "Zindagi mein sabhi pyar kiya kartein hain" (written by Qateel Shifai); "Dekh tu dil ki jaan se uthta hai" (by Mir); "Shola tha jal bujha hoon"; "Yeh mojeza bhi mohabbat dikhaye mujhe" (by Qateen Shifai); "Abke bicchde khwaabon mein mile" (Ahmad Faraz); "Baat karni mujhe mushkil" (Bahadur Shah Zafar); Uzr aane mein bhi hai (Daagh Dehlvi)
Born to a family of Kalawant musicians in Luna village, now within the Indian state of Rajasthan, Hassan was musically baptised by father Ustad Azeem Khan and uncle Ustad Ismail Khan, who were Dhrupad musicians.
The family migrated to Pakistan after partition and sank into penury. Eking out a living was difficult. Young Mehdi started to work in a bicycle shop and later became an auto-mechanic.
In his book "Mehdi Hasan: The person & his Music", Pakistani author Asif Noorani has highlighted this phase of Hassan's life and written that his humility stood tall against the celebrity and greatness he had achieved later.
"He had earned his living by repairing automobiles during his younger days. During his years of stardom, his harmonium broke and he started repairing it himself, wittingly replying to the folks surrounding him that this was a work of cake in comparison to the number of engines that he had repaired within the past," Noorani wrote.
The hardships of life notwithstanding, Hassan stuck to his music and continued along with his "riyaz". His patience paid off when he was invited to sing for Radio Pakistan in 1957 - first as a 'thumri' singer after which as a 'ghazal' exponent.
Hassan needed to work harder than lots of his younger colleagues but his innovative approach earned him fame.
Traditionally, ghazals were sung in a thumri-like manner. They were set to ragas like Khamaj, Piloo and Desh. The classical format stymied the scope of the compositions - preventing it from innovating.
However, Hassan pioneered a "ghazal gayaki" that played upon the mood of the music in preference to at the classical nuances.
A composer of rare brilliance, he had an inherent ability for using the ragas most suitable to the ghazal's appeal.
Hassan's style combined dhrupad, khayal and Rajasthani folk music.
His magic was not confined to Pakistan, he was equally popular in India. In fact, he was some of the first Pakistani ghazal singers who charmed Indian audiences.
His fan following had impressive names - Lata Mangeshkar once described his voice because the "Voice of God" and previous prime minister Atal Bihari Vajpayee was an excellent fan of his melodies.
In October 2010, Mangeshkar and Hassan teamed up for "Tera Milana" within the album "Sarhadein", probably their first and last duet.
In 1978, when Vajpayee was foreign minister, Hassan had performed at his residence.
After shining at the music firmament from 1962 to the early 1980s, Hassan's career started fading as illness took a toll and Urdu films became less popular.
His home country honoured him with several awards and honours - Tamgha-e-Imtiaz, Pride of Performance and Hilal-e-Imtiaz.
The Nepal government too honoured him with the Gorkha Dakshina Bahu.
