Brennan travels in search of countries and languages whose music is under-represented internationally, making field recordings of musicians and producing albums of their work.

[23] He received two Grammy Award nominations for producing albums in the traditional folk category (Ramblin' Jack Elliott's I Stand Alone in 2006,[6] and Peter Case's Let Us Now Praise Sleepy John in 2007[7]).

The Ramblin' Jack record features Lucinda Williams[24] and members of Wilco,[25] X, Los Lobos, and the Red Hot Chili Peppers.

[26] In 2009, he and his wife, the Italian-Rwandan filmmaker,[27] photographer,[28] and author,[29] Marilena Umuhoza Delli, began traveling the world in search of countries and languages that were underrepresented internationally.

[42][43] In 2011, he won a Grammy Award[4][44] for the Tuareg band, Tinariwen's Tassili album, which was recorded live in the southeast Algerian desert just months before the Arab Spring erupted and war swept through the area.

[95] Through word of mouth, he began teaching full-time at hospitals, clinics, jails and schools in the San Francisco Bay Area and greater California.

This teaching eventually led him around the country and then the world, having now taught in Africa,[96] Southeast Asia, Europe,[97] Australia,[98] and the Middle East, at such places as University of California, Berkeley,[99] the Betty Ford Center,[100] and the National Accademia of Science (Rome).

[102] In the fall of 2019, Brennan produced a "sonic memorial" album featuring voices and songs from the homeless community of West Oakland.

[103] Brennan wrote a piece in May 2019 for the Chicago Tribune criticizing the racist and misogynistic lyrics of The Rolling Stones', "Brown Sugar", and calling for the band to cease playing it live.

[105][106] At age 19, Brennan's poetry was published for the first time in an anthology (Fineline Thunder)[107] curated by his adult-school creative writing workshop instructor, Betty Solomon.

"[136] Small Press Picks noted, "In vividly re-creating Kristian's personal journey, Brennan offers a layered and moving exploration of the truth…"[137] His fourth book was How Music Dies (or Lives): Field-recording and the Battle For Democracy in the Arts.

[138][139] In it he explores concerns related to the continuing domination of English language media across the planet,[140][141] and details how recording technology can lead to more lifeless results as well as centralization of content.

Brennan has hosted book events with the disability rights activist Judith E. Heumann[146] (featured in the Academy Award nominated documentary Crip Camp); tech visionary Jaron Lanier;[147] David Harrington (Kronos Quartet);[148] feminist scholar Silvia Federici;[149] Ted Hughes Award winning, deaf poet, Raymond Antrobus;[150] crime novelist Gary Phillips (writer);[151] and music producer Joe Boyd (Nick Drake, Billy Bragg, Toots and the Maytals).