Gerasimov, 67, is the Russian military's highest-ranking official after Defence Minister Sergei Shoigu and has access to nuclear codes.
Surly and unsmiling, he is often seen briefing Putin alongside Shoigu, who has no military background, during drills and meetings.
Observers believe he was closely involved in devising the plan to send troops to Ukraine in conditions of absolute secrecy.
After taking over in 2012, Gerasimov has been the longest-serving chief of the general staff of the post-Soviet era.
During his tenure he saw the Kremlin annex Crimea from Ukraine in 2014 and intervene in Syria to buttress the regime of Bashar al-Assad the following year.
Presenting Gerasimov to Putin in 2012, the defence minister described him as "a military man to the roots of his hair" and pointed to his "colossal" experience, both at the General Staff and in the field.
Gerasimov oversaw operations in Moscow's second war in Chechnya during his time in the 58th Army of the North Caucasus Military District between 1998 and 2003.
Attempt on Gerasimov's life
At the beginning of the offensive Gerasimov was believed to be closely involved in operations on the ground in Ukraine, and a Pentagon official said the general had visited the eastern region of Donbas in spring.
Gerasimov was thought to be touring the front to better understand field conditions and rally his troops after Russian forces failed to capture Kyiv.
An adviser to Ukraine's president, Oleksiy Arestovych, said in December that Ukrainian troops targeted Gerasimov during his visit to eastern Ukraine but he managed to escape.
On 9 May, Gerasimov was conspicuously absent from Russia's annual Victory Day parade celebrating the Soviet Union's victory over Nazi Germany in the Second World War in 1945.
The Institute for the Study of War, a Washington-based think tank, expressed doubt that Gerasimov would be able to claw back a victory after fading "into obscurity following the collapse of Russia's flawed initial planning assumptions".
"Gerasimov is highly unlikely to successfully meet Putin's unrealistic expectations for his performance."
As commander of Russian armed forces in Ukraine, Gerasimov replaced Sergei Surovikin, who had had the top job for three months only and will now serve as one of Gerasimov's deputies.
The name of Surovikin's predecessor was not publicly announced, and the entire command structure during the offensive remains opaque.
"Gerasimov will likely preside over a disorganised command structure plagued by endemic, persistent, and self-reinforcing failures that he largely set into motion," the Institute for the Study of War said.
Many analysts also pointed out that by thrusting Gerasimov into the top role at the front, Putin wanted to reassert control after irregular forces fighting in Ukraine including mercenary group Wagner praised Surovikin and criticised the Russian army.
Born in Kazan, Gerasimov graduated from the Kazan Higher Tank Command School in 1977 and quickly climbed through the ranks. He commanded the Leningrad Military District from 2007 to 2009 and the Moscow Military District from 2009 to 2010.
General Pierre de Villiers, who was chief of the general staff of the French armed forces between 2014 and 2017, said he remembered Gerasimov as a no-nonsense military man.
"What I noticed was that he only recognised one thing, the balance of power," he said on French television.