Prince Hubertus of Prussia
Prince Hubertus of Prussia (Hubertus Karl Wilhelm; 30 September 1909 – 8 April 1950) was the third son of Crown Prince Wilhelm of Germany and Duchess Cecilie of Mecklenburg-Schwerin.
Henry Borwin II, Lord of Mecklenburg
Henry Borwin II, Lord of Mecklenburg( 1170 – 5 June 1226) was a member of the House of Mecklenburg, was a Prince of Mecklenburg from 1219 to 1226 and Lord of Rostock( 1225- 1226).
Borwin, Duke of Mecklenburg
Borwin, Duke of Mecklenburg( given names:" Georg Borwin Friedrich Franz Karl Stephan Konrad Hubertus Maria"; born 10 June 1956) has been the head of the House of Mecklenburg- Strelitz since 1996 and of the entire House of Mecklenburg since 2001.
The death of Friedrich Franz, Hereditary Grand Duke of Mecklenburg- Schwerin the last male member of the House of Mecklenburg- Schwerin on 31 July 2001 made Strelitz the only remaining line of the House of Mecklenburg, which ruled in Mecklenburg until 1918.
Borwin and his sons, Alexander and Michael, are the only known surviving legitimate male- line descendants of the medieval princely dynasty descended from Niklot of the Obotrites, which has included King Albert of Sweden.
Duke William of Mecklenburg-Schwerin
Willem of Mecklenburg-Schwerin (Ludwigsburg, 5 March 1827 - Heidelberg, 28 July 1879), was the second son of Hereditary Grand Duke Paul Friedrich of Mecklenburg and his wife Princess Alexandrine of Prussia, daughter of Frederick William III.
Princess Alexandrine of Prussia (1803–1892)
Princess Alexandrine of Prussia (23 February 1803 – 21 April 1892) was Grand Duchess of Mecklenburg-Schwerin by marriage to Paul Frederick, Grand Duke of Mecklenburg-Schwerin.
She was the daughter of Frederick William III of Prussia and Louise of Mecklenburg-Strelitz.
Georg Alexander, Duke of Mecklenburg
Georg Alexander, Duke of Mecklenburg( 27 August 1921 – 26 January 1996) was the head of the House of Mecklenburg- Strelitz from 1963 until his death.
John VI, Duke of Mecklenburg
John VI, Duke of Mecklenburg( 1439 – 1474) was a Duke of Mecklenburg.
John Albert I, Duke of Mecklenburg
John Albert I, Duke of Mecklenburg, in older literature known as" John" or" Johann"( 23 December 1525 – 12 February 1576), was the reigning Duke of Mecklenburg- Güstrow from 1547 to 1556 and of Mecklenburg- Schwerin from 1556 to 1576.
In 1549 John Albert I saw to it that the parliament of Mecklenburg carried through the Reformation for the entire Duchy.
Duchess Gustave Caroline of Mecklenburg-Strelitz
Duchess Gustave Caroline of Mecklenburg- Strelitz( 12 July 1694 – 13 April 1748) was a daughter of Adolphus Frederick II, Duke of Mecklenburg and Princess Maria of Mecklenburg.
Prince Wilhelm of Prussia (1906–1940)
Prince Wilhelm of Prussia (Wilhelm Friedrich Franz Joseph Christian Olaf; 4 July 1906 – 26 May 1940) was the eldest child of Crown Prince Wilhelm of Germany and Duchess Cecilie of Mecklenburg-Schwerin.
At his birth, he was second in line to the German throne and was expected to succeed to the throne after the deaths of his paternal grandfather and father.
Both of whom, however, outlived him.

Question: Who is the maternal grandfather of Duke William Of Mecklenburg-Schwerin?
Answer: Frederick William III of Prussia