He was born at Klein-Flottbeck, Holstein (now part of Altona, Hamburg). His father, Bernhard Ernst von Bülow, was a Danish and German statesman and member of the Bülow family. His brother, Major-General Karl Ulrich von Bülow, was a cavalry commander during World War I who took part in the attack on Liège in August 1914. Bülow attributed his grasp of English and French to having learnt it from governesses as a young child. His father spoke French, and his mother spoke English, as was common in Hamburg society.
In 1856, his father was sent to the Federal Diet in Frankfurt to represent Holstein and Lauenburg, when Otto von Bismarck was also there to represent Prussia. He became a great friend of Bismarck's son Herbert when they played together. At 13, the family moved to Neustrelitz when his father became Chief Minister to the Grand Duke of Mecklenburg, where Bernhard attended the Frankfort gymnasium, before attending Lausanne, Leipzig and Berlin Universities.
He volunteered for military service during the Franco-Prussian War and became a lance-corporal in the King's Hussar Regiment. In December 1870, the squadron was in action near Amiens, and he later described charging and killing French riflemen with his sabre. He was promoted to lieutenant and was invited to remain in the army after the war but declined. He completed his law degree at the University of Greifswald in 1872. Afterwards, he entered first the Prussian Civil Service and then the diplomatic service.
In 1873 his father became Secretary of State for Foreign Affairs in the German government, serving under Bismarck. Bülow entered the diplomatic corps. His first short assignments were to Rome, St. Petersburg, Vienna and then Athens. In 1876, he was appointed attaché to the German embassy in Paris, attended the Congress of Berlin as a secretary and became second secretary to the embassy in 1880.
In 1884, he had hoped to be posted to London but instead became first secretary at the embassy in St. Petersburg. On the way to his new assignment he stayed for a couple of days at Varzin with the Bismarck family. Bismarck explained that he considered relations with Russia much more important than Britain, which was why he had posted Bülow there. Bismarck reported himself impressed by Bülow's calmness and demeanour during the interview. In Russia, he acted as chargé d'affaires in 1887 advocating ethnic cleansing of Poles from Polish territories of the German Empire in a future armed conflict. Bülow wrote regularly to the Foreign Office, complaining about his superior, Ambassador Schweinitz, who, however, was well-liked. Bülow earned for himself a reputation as only a schemer. In 1885, Holstein noted that Bülow was attempting to have Prince Chlodwig von Hohenlohe-Schillingsfürst removed as ambassador to France to get the post, all the while exchanging friendly letters with him.