Angus Young later revealed that he realised during the recording of the 2008 album, Black Ice, that his brother's faculties were impaired.
Malcolm had been diagnosed with lung cancer that year. He received early treatment, but his health problems continued when doctors discovered he had a heart condition that required a pacemaker. Then dementia struck.
"It was like everything hit him at once," Angus Young said.
"The physical side of him, he got great treatment for all that so he's good with all that, but the mental side has deteriorated. He himself has said, 'I won't be able to do it any more'."
Young took a leave of absence from the band in April 2014, and in September announced his retirement. He urged the band to continue touring and making music.
Steve Young, his nephew, replaced Malcolm in the line-up for AC/DC's next album, Rock Or Bust.
Malcolm Mitchell Young was born in Glasgow, Scotland, on January 6, 1953, one of the six children of William and Margaret Young.
When Malcolm was 10 the family migrated to Australia, settling into a single- storey semi at 4 Burleigh Street, Burwood, in Sydney's inner west. (The house is now on the National Trust register.)
Music ran through the six siblings: oldest brother Alex was a musician in The Big Six, and George became a member of The Easybeats, co-writing hits, including Friday On My Mind.
The Young home was sometimes besieged by young female fans, and Malcolm and Angus soon decided that they, too, wanted to enter the music industry.
Both brothers attended Ashfield Boys' High School (the uniform of which Angus would later make famous). After leaving school at 15, Malcolm got a job maintaining sewing machinea for a bra factory and joined a local band called the Velvet Underground (no relation to Lou Reed's outfit).
After the Easybeats's ended in 1970, George focused on songwriting and producing. One of the studio groups he formed in 1973, the Marcus Hook Rock Band, included Malcolm and Angus.
In 1973 Malcolm invited Angus to join a new band he was forming.
"I was amazed when he asked me to come down to a rehearsal and play," Angus said in a 1992 interview. Until then, he added, the brothers had worked separately, with Malcolm "in one room with his tape recorded putting tunes together, and I would be in the other room pretending I was Jimi Hendrix".
The band's name was supplied by their sister Margaret, who noticed the letters AC/DC on a sewing machine. She also suggested the diminutive Angus wear his school uniform on stage.
Singer Bon Scott, drummer Phil Rudd, and bassist Mark Evans. AC/DC first appeared on Countdown in April 1975, performing Baby Please Don't Go.
Their first four albums were produced by brother George and his Easybeats colleague Harry Vanda.
The following February AC/DC recorded It's A Long Way To the Top on the back of a moving flat-bed truck driving down Melbourne's Swanston Street. They did their first world tour later that year.
More international tours followed in 1977, 1978 and 1979. On February 19, 1980, Scott was found dead of asphyxiation after choking on his own vomit after an all-night drinking binge in London. The rest of the band travelled to the Bahamas to regroup, recover and record Back In Black. Brian Johnson was drafted in to replace Scott.
Back In Black sold 50 million copies worldwide, and remains one of the biggest selling albums of all time.
The two brothers wrote and recorded together. Angus may have had a higher profile as AC/DC's eternal schoolboy, but Malcolm's solid work on rhythm guitar gave the band its musical backbone.
Malcolm was widely seen as the brains of the band, both in a business sense and musically.
A critic in The Guardian once described the essence of Malcolm Young's contribution to AC/DC: "Malcolm Young understood that a great riff does not need 427 components to make it great, that what it really needs is clarity.
That meant stripping riffs down rather than building them up, and it also meant understanding volume. Given how loud AC/DC can be in concert - ear-ringingly, sternum-shakingly loud - it might be surprising to learn that, in the studio at least, Malcolm Young favoured quietness: he played with his amps turned down, but with the mics extremely close.
That's why, on the great AC/DC albums, you hear not just the chords of the riffs, but their very texture, their burnished, rounded sound. It's why AC/DC are immediately recognisable, whether or not you know the song."
Angus Young once told Guitar Player magazine that he could not fill Malcolm's shoes as a guitarist, but Malcolm could fill his.
When AC/DC toured Australia in 1981 - for the first time in four years - even The Australian Women's Weekly knew they were something special: "These boys have rock in their veins; music isn't an art for them, it's a lifestyle."
Perhaps too much so. Malcolm, always a heavy drinker, took leave in 1988 to dry out. His nephew Stevie Young took his place on the Blow Up Your Video world tour.
Soon after Malcolm returned to the band it recorded one of its most successful albums, The Razor's Edge.
In 1991 three teenage fans were crushed to death at an AC/DC concert in Salt Lake City when the crowd surged forward. The band played on for 20 minutes, unaware of what was happening. Afterwards they extended their sympathy to the families and stated that "nothing anyone can say or do will diminish the tragic loss or sense of grief".
The band continued its pattern of recording albums and doing world tours: Ballbreaker (1996), Stiff Upper Lip (2000-2001); Black Ice (2008-2010).
They were inducted into the ARIA Hall of Fame in March 2003. In 2009 AC/DC topped BRW's list of Australia's top-earning entertainers, displacing The Wiggles.
A street in Leganes, near Madrid, was named Calle de AC/DC in 2000, Melbourne bestowed a similar honour in 2004, changing Corporation Lane to ACDC Lane.
In 2007 AC/DC sold 1.3 million CDs in the US - even though they had not released a new album for seven years.
Rolling Stone magazine has called them "one of the most enduringly popular hard- rock bands on the planet".
After Malcolm's retirement in 2014, lead singer Brian Johnson told the ABC's 7.30 program: "It was a strange feeling because your work mate, you worked with for the last, for me 35 years wasn't there any more."
In 2015 Young moved into the dementia care unit of Lulworth House, in Sydney's Elizabeth Bay.
He is survived by his wife, Linda, and their children Ross and Cara.
