Keil produced a vast and diversified work that included painting, drawing and illustration, azulejo tiles, graphic and furniture design, tapestry and scenography.

Throughout her life she engaged in many areas, including painting and drawing, illustration, graphic arts, printmaking, tiles, tapestry, furniture, decoration, scenography and costumes.

In 1940, she designed sets and costumes for the ballet Lenda das Amendoeiras, presented at the debut show of the Companhia de bailado Verde Gaio.

Of her vast production, the tile panel O mar, on Avenida Infante Santo, Lisbon, and the extensive collaboration with the Metropolitano de Lisboa can be highlighted.

Oscillating between the direct, simple and immediate image, and the fusion of spaces or even the subtle surrealization of the narrative, Maria Keil starts "from a real situation, […] takes from her only what already brings her a touch of unreality.

"[10] "The graphic process of over-articulation of planes becomes a compositional structure in painting, capable of making the contours of the linear shape coincide with the limits of the flat color, through the relation of plastic equivalences.

They represent an extension of the attempts of renovation of the Portuguese tile carried out by Jorge Barradas, Carlos Botelho, Bernardo Marques or Fred Kradolfer.

"[11] "Maria Keil did not bet on the renovation of the azulejo from a mere vocabulary change, because she invented another language for the tile, from a methodical construction of spatial-optical effects.

With a clearly symbolic chromatism where blues and greens predominate, this work matches the figurative allusions (the image of the fisherman with his son, boats, shells ...) with the markedly decorative tendency of the entire panel, dominated by geometric patterns: "her cultural reference is not located in the pictorial panels, historians or naturalists of erudite production, but in its determinant borders where the possibilities of geometry and colour are transmuted into rhythms.

"[13] For the vast work for the Metropolitano de Lisboa (1957 – c. 1972), she will opt for strictly abstract forms, experimenting with variations where shapes inherited from the history of the tile intersect with abstract elements that may originate, for example, in Neoplasticism: "languages and values, Maria Keil senses the path of a contemporary condition that reuses successive and disparate poetics as operative signs, aiming at the absolute availability of the interplay of forms and colors.