Born 18 January 1726 he was fourteen years younger than his brother Friedrich II, later known as King Frederick the Great. When their father died in 1740, Frederick behaved as a strict parent
towards his younger brothers and sisters. Heinrich was always envious of his eldest brother and as a small boy at times refused to speak to him for months. Friedrich then complained, maintainging that they lived together like a dreary old married couple and asked him to try to overcome his dislike. Both brothers had a marvellously logical mind,much energy and loved French literature, so that Heinrich was regarded as a caricature of Friedrich.
On 25 June 1752 Heinrich married Wilhelmine of Hessen-Darmstadt, a marriage which remained childless. Heinrich's indifference to her was, according to Friedrich, due to a suspected affaire with Countess Bentinck (nee Countess Charlotte Sophie von Aldenburg) who was in Berlin for a lawsuit against her estranged husband. However, later on Heinrich was regarded to be a homosexual.

In 1756 Friedrich was concerned about a possible war with France and the Empress Maria Theresa. Although his advisers urged him to do nothing, Winterfeldt provided proof that the King of Saxony was ready to join the Imperial forces and suggested to attack as, with Saxony as
an enemy, Prussia would be vulnerable. Heinrich, fearing for his beloved France, was also against this war.
Friedrich asked Empress Maria Theresa for her intentions and requested a straightforward answer, not something from an oracle. As the answer given on 26 August was still evasive, two days later
Friedrich marched on Dresden, which was the beginning of the Seven Years' War. Meanwhile, Heinrich and Friedrich had gone first to Berlin to say goodbye to their mother.
Heinrich assisted his brother greatly, at one stage plunging himself waist-deep into a bog to storm a battery. A remarkable soldier, he defended Dresden against the Imperial forces. During his stay there, he was visited by Friedrich and, in harmony with each other, they were united in their concern for their desperately ill sister Wilhelmine.
In 1759 their armies had lost most of the veterans and the young recruits were much rougher to civilians. This both Friedrich and Heinrich, more humane and sensitive, tried to prevent. In 1760 the relationship between Heinrich and Friedrich had again changed; Heinrich would try to quarrel and Friedrich would behave as though nothing had happened. At the same time, the king wrote to their sister Amalie: "Heinrich performs the impossible. I must say I truly love him. He has wit and ability, both very rare. I depend upon him."
At the end of 1761 the situation for the Prussians had become desperate. When Heinrich lost the Saxon mountains, so little territory remained to him that he could hardly feed his troops. In early 1762 they expected Prussia to be overrun by Russian forces, but the death of the Russian Empress Elisabeth caused a change, as her nephew and heir was an admirer of Frederick the Great.

France, ruined through the war, refrained from further action. The only major engagement that summer was when Heinrich decisively defeated the Imperial general, the Prince of Stolberg. According to Friedrich: "You alone have the glory of breaking down Austrian obstinacy."
After the Seven Years' War, Heinrich gave his childless wife a wing of his Berlin palace to live in, forbidding her to visit him at Rheinsberg. During the following years Heinrich and Friedrich became
much closer, though Heinrich would still say dreadful things about his elder brother. Nevertheless Friedrich built a palace for Heinrich in Berlin and gave him properties, making Heinrich a very rich man. In the winter of 1770 Heinrich went to Russia to visit Catherine the Great, whom he had known as a child. They got on very well and, a few months later, Russia together with Austria and Prussia agreed to a division of Poland. 
At the end of his life Friedrich told Heinrich that he loved his country too much to say 'apres mois le deluge' and that he would do anything he could until his dying day. This was said because the king was very much concerned about his successor. On 17 August 1786 Frederick the Great died, Heinrich surviving him until 3 August 1802.
 

Source: Leo van de Pas