A good place to start would be with the world's most well-known "Mother" artwork, that of by James McNeill Whistler, painted in 1871:
above: James McNeill Whistler, Whistler's Mother 1871, Musée d'Orsay, Paris
The following portraits are presented in chronological order from the earliest to the most recent. While it's true that many of these artists painted multiple sittings of their mothers (e.g. Lautrec, Cassat, and Cezanne), I chose to share those I found to be the most compelling.
Albrecht Durer, portrait of Barbara Durere, the artist's mother, 1490:
Guido Reni, Portrait of the Artist's Mother, 1612:
Rembrandt, Portrait of the Artist's Mother, 1630:
Sir Thomas Lawrence, Portrait (sketch) of Lucy Lawrence, the Artist's mother, 1797:
Alfred Rethel, portrait of the Artist's Mother, 1836:
....
Camille Corot, Madame Corot, the Artist's Mother, Born Marie-Francoise Oberson, 1838:
Pierre Renoir, Portrait of the Artist's Mother, 1860:
Paul Cezanne, Portrait of the Artist's Mother, 1866-67:
Mary Cassat, the Artist's Mother reading Le Figaro, 1878:
Edouard Manet, Mother in the garden at Bellevue, 1880:
Georges Seurat, Aquatint of the artist's mother, 1883:
Henri de Toulouse-Lautrec, Countess Adele Zoe de Toulouse Lautrec, The Artist's Mother, 1883:
Vincent Van Gogh, Portrait of the Artist's Mother, October, 1888:
Paul Gauguin, Portrait of Aline Gauguin, 1890:
Pablo Picasso, Portrait of the Artist's Mother, 1896:
Edvard Much, The Dead Mother (the artist's mother died when he was five from tuberculosis), 1899:
Franz Marc, portrait of his mother, 1902:
Georgio di Chirico, portrait of the artist's mother, 1911:
Egon Shiele, Mother Sleeping, 1911:
Juan Gris, portrait of the artist's mother, 1912:
Edward Hopper, Elizabeth Griffiths Smith Hopper, the artist's mother, 1916:
Salvador Dali, portrait of the artist's mother, 1920:
Norman Rockwell's Mother Tucking Children Into Bed (for which his mother Irene was the model), 1921:
Grant Wood, Woman With Plants (his mother), 1929:
Arshile Gorky, portrait of the artist's mother, 1936:
William H. Johnson, Mom Alice, 1944:
Alice Neel, My Mother, 1952:
Lucian Freud, The Painter's Mother, 1973:
Andy Warhol, silkscreen of his mother, Julia Warhola, 1974 :
David Hockney, Mum, 1985:
Daphne Todd, Last Portrait Of Mother, 2009:
David Kassan, portrait of the artist's mother, 2010:
