The Mask series, initiated in the early 2000s, continues to employ a rich color palette and themes of depth and perspective that were foundational in the Butterfly series. However, the Mask works distinguish themselves through their rounded, organic forms and lines, presenting a more figurative and flowing aesthetic. This series represents a departure from the hard edges and sharp geometric patterns of his earlier works, embracing instead a more intuitive, sculptural form of expression.

Grotjahn’s work has been widely recognized and exhibited in prominent institutions worldwide, such as the UCLA Hammer Museum in Los Angeles, the Whitney Museum of American Art in New York, and the Museum of Modern Art in New York. His art is held in numerous prestigious collections, including the Broad Art Foundation in Santa Monica, the Hammer Museum in Los Angeles, and the Carnegie Museum of Art in Pittsburgh.

Despite his success, Grotjahn has not been without controversy, notably involving a lawsuit over artist royalties. In 2011, he sued a collector to recover a 5% royalty for artworks that were resold, highlighting the challenges artists face in the secondary market. This case settled in 2012, underscoring Grotjahn's assertiveness in protecting his rights and interests as an artist.

Grotjahn's Mask series, with its rich exploration of form and color, marks an important phase in the artist's career, showcasing his ability to transition between different artistic expressions while maintaining a deep engagement with the formal and thematic concerns that have defined his work​ (Wikipedia)​​ (Squarespace)​​ (Berggruen)​.